Whether or not IC is better will depend on which weapon you're using and what fortification the target has. In your analysis, you're using a weapon with a (base) crit range of 19-20, and a x3 multiplier. That will basically work only for khopeshes, which have the best combination of crit range and crit multiplier. For other weapons, IC won't perform as well. Also, once there's fortification, IC will also not perform as well. Also, some people choose to go with mineral 2 weapons which have keen, and thus, don't need the IC feat.
However, it's true that IC may perform better than GTWF (or, for that matter, ITWF), depending on the circumstances. The TWF feats basically increase your offhand attacks, but they are weaker than your primary attacks (due to only getting half of the strength bonus). How much weaker will depend on your specific build and which buffs are available, but say it's 15% less damage than your main hand (i.e. offhand damage = 0.85 * main hand damage). Also, say you're not using seeker damage (so that relative ratios can be used). Assuming khopeshes vs 0% fort, without IC across 20 hits, you'll do an average of (17*1 + 2*3)/20 = 23/20 of the base damage. With IC, you'll do an average of (15*1 + 4*3)/20 = 27/20 of the base damage. Thus, IC improves your original damage by a ratio of 27/23. Taking all this together, the damage output that you do, relative to the average damage of a main hand hit (including crits but without the IC feat), is:
Code:
Feats No_IC ImpCrit
None 1.17 1.373
TWF 1.34 1.573
ITWF 1.51 1.773
GTWF 1.68 1.972
Notice that without IC, the damage is somewhat less than the 20% per tier gains from the TWF feats, because offhand damage will be somewhat less than your main hand damage (here, assumed 15% less). So yeah you'd benefit more from taking the IC feat if you're going khopeshes, vs 0% fort. If you're going dwarven axes, however, it would look like:
Code:
Feats No_IC ImpCrit
None 1.17 1.281
TWF 1.34 1.468
ITWF 1.51 1.654
GTWF 1.68 1.840
and it becomes better to take the TWF feats first. So it really depends on what weapons you choose, and the crit profile they have. If you're using rapiers/scimitars/falchions (crit range 18-20, crit multiplier 2), it would look like:
Code:
Feats No_IC ImpCrit
None 1.17 1.330
TWF 1.34 1.523
ITWF 1.51 1.716
GTWF 1.68 1.909
They're basically tied. In this case, however, the TWF would win out as soon as fortification is introduced.
What's really happening here is that the TWF feats are adding a fixed amount of damage output per tier, while IC multiplies your existing damage output by a certain amount. So for the TWF feats, adding 0.17x more damage, in going from no feats to TWF, means 1.34/1.17 = 1.145x or 14.5% more damage. Adding the same 0.17x more damage when you already have ITWF (1.51x), however, means 1.68/1.51 = 1.113 or 11.3% more damage. So each TWF tier is worth slightly less than the previous one in terms of your overall damage output, so basically diminishing returns, while IC is a flat increase, which depends on the weapon's crit profile (crit range and crit multiplier). Whether IC is better will depend on your weapon choice. For khopeshes, it will likely be better vs 0% fort. For handwraps, you'd want the TWF feats instead.