Some musing on the genetically enhanced soldiers.
You'll almost always want them in heavy power armor - you're paying more for the extra hp than you would pay for the extra armor, and there aren't really many situations where the hp is better (Heavy crew served anti-personnel being the most notable exception, but light bombardment is the other way around so it kind of cancels out), so it makes sense to increase the cheaper armor first.
So I expect if you use genetically enhanced infantry, you want space marines - 2x hp 2x armor, once you have all the genetic engineered infantry tech. You'll be paying five times as much for one as for an unarmored infantry, but you'll get extremely high resistance to small arms. This makes them a natural for boarding combat, since there you'll probably be mostly worried about light personal weapons from the crew.
Doing the math, assuming near equal tech the most efficient weapons against space marines would be light anti-vehicle or medium autocannons (both get the same kills per ton of weapon). Still decent counters are, in descending efficiency, heavy bombardment, light autocannon, Medium bombardment & Heavy Crew Served anti-personnel, and then heavy autocannon. Basically you pay 5x as much for infantry that are extremely resistant to light weapons but still take the same losses to anti-tank or heavy weapons - not a bad deal.