IO Enhancement Strength

How good are they compared to what we have already?

To answer that it's probably easiest to just show you a table, which shows how strong IOs are at each level. IOs have a fixed enhancement value that is tied to the level of the enhancement itself - the level of your character is irrelevent (as long as you are at least -3 to the enhancement so you can slot it). IOs also never expire, and you can slot a much lower IO in your high level character at anytime too (e.g. a Level 30 IO in a level 50 character).

Code:
Level Sched B Sched A Sched C Sched D
---------------------------------------
10 0.07 0.117 0.14 0.21
11 0.08 0.133 0.16 0.24
12 0.09 0.15 0.18 0.27
13 0.1 0.167 0.2 0.3 <-- Even-level DO Value
14 0.108 0.179 0.215 0.323
15 0.115 0.192 0.231 0.346 <-- +3 DO Value
16 0.123 0.205 0.246 0.369
17 0.131 0.218 0.262 0.392
18 0.138 0.231 0.277 0.415
19 0.146 0.243 0.292 0.438
20 0.154 0.256 0.308 0.462
21 0.162 0.269 0.323 0.485
22 0.169 0.282 0.338 0.508
23 0.177 0.295 0.354 0.531
24 0.185 0.307 0.369 0.554
25 0.192 0.32 0.385 0.577
26 0.2 0.333 0.4 0.6 <-- Even-level SO Value
27 0.202 0.337 0.405 0.607
28 0.205 0.341 0.409 0.614
29 0.207 0.344 0.414 0.62
30 0.209 0.348 0.418 0.627
31 0.211 0.352 0.423 0.634
32 0.214 0.356 0.427 0.641
33 0.216 0.359 0.432 0.648
34 0.218 0.363 0.436 0.655
35 0.22 0.367 0.441 0.661
36 0.223 0.371 0.445 0.668
37 0.225 0.375 0.45 0.675
38 0.227 0.378 0.455 0.682
39 0.23 0.382 0.459 0.689 <-- +3 SO Value
40 0.232 0.386 0.464 0.695
41 0.234 0.39 0.468 0.702
42 0.236 0.394 0.473 0.709
43 0.239 0.397 0.477 0.716
44 0.241 0.401 0.482 0.723
45 0.243 0.405 0.486 0.73
46 0.245 0.409 0.491 0.736
47 0.248 0.412 0.495 0.743
48 0.25 0.416 0.5 0.75
49 0.252 0.42 0.505 0.757
50 0.255 0.424 0.509 0.764
51 0.257 0.428 0.514 0.77
52 0.259 0.431 0.518 0.777
53 0.261 0.435 0.523 0.784

Note: all enhancement types are Schedule A, except for:

Schedule B: Defense, Damage Resistance, ToHit Buff, ToHit DeBuff, Range
Schedule C: Interrupt Reduction
Schedule D: Knockback

Bleurghh - Numbers! Eh? What does that actually mean?

Well basically at level 12, when you can first buy DOs, you are also able to slot Level 15 IOs, which are already the same strength as +3 DOs and remain that strong forever (or until you replace them with something better).

At level 22, when you can first buy SOs, the equivalent IOs are slightly weaker than an even-level SO (and the SOs you can buy at this level will be +3 of course), however the IOs don't expire - so you may still prefer to get/craft IOs anyway.

+3 SOs remain stronger than equivalent IOs until you reach level 37, when you can slot Level 40 IOs, which are stronger than a +3 SO, remain that strong as you level, and never expire either.

It should also be noted that the "Basic" IOs (i.e. the IOs that enhance just one aspect of a power and aren't part of a Set) are only available in 5 level increments - even when they drop from mobs.

OK - So I get the Basic IOs, but what about these Set ones?

The "Set" IOs (which are part of named sets and tend to enhance more than one aspect of a power) use the same table above, but the enhancement value of each aspect is reduced according to this pattern:

100% per aspect for single aspect IOs (equivalent IO strength: 1)
62.5% per aspect for dual aspect IOs (equivalent IO strength: 1.25)
50% per aspect for triple aspect IOs (equivalent IO strength: 1.5)
43.75% per aspect for quad aspect IOs (equivalent IO strength: 1.75)

So for example a level 50 ACC/DAM Set IO would give 62.5% of 0.424 (0.265) for both ACC and DAM, whereas a level 50 END/DEF/RECH would give 50% of 0.424 (0.212) for END and RECH, but 50% of 0.255 (0.128) for DEF (because Def is Schedule B).

The equivalent IO strength is an indicator of the 'slot value' of each type of IO. For example slotting four triple aspect IOs would be the equivalent of slotting 1.5 + 1.5 + 1.5 + 1.5 = 6 IOs. I.e. you would get 2 extra slots-worth of enhancement for free as it were. For comparison, multi-aspect HOs operate at full SO strength for each aspect, so they have equivalent SO strength of 2 or 3, depending on whether they are dual or triple aspect - which is why HOs will remain better than equivalent IOs in most cases.

Set IOs also grant character-wide Set Bonuses when more than one of any particular Set is slotted in a power, but that is for another guide!

What about ED? Can I still 3 slot stuff?

Well IOs follow exactly the same rules as other enhancement types - afterall it is overall enhancement value that ED works on, not the number of enhancements per se. The "rule of thumb" for SOs has always been that 3 slotting is the most sensible 'maximum' slotting for any one aspect with ED. After that any additional enhancement is at a very poor rate and probably not worthwhile.

In fact powers that you currently 3 slot with SOs are probably the ones least likely to benefit from IOs if you just want to replace like for like. At the moment 3 even level Schedule A SOs in a power will grant you 0.95 enhancement after ED (and 3 +3 SOs is 0.972). Replacing those with 3 Level 50 IOs will grant you only 0.991 after ED - barely any improvement at all.

However, it should be worth noting that if you 2 slot a power with Level 50 IOs, you will be getting 0.833 enhancement after ED (compared with 0.667 from 2 even level SOs) - which may be worthwhile enough for you, and freeing up a slot for elsewhere in your build.

Alternatively you can look at it the other way around; to achieve the same overall enhancement from 3 slots with IOs as you do with even level SOs, you can do so with just level 30 IOs (which yields 0.957 after ED). You may never need to bother upgrading those IOs in a 3 slotted power, as the most you are going to manage is about an additional 3.4% enhancement strength overall.