[spoiler=rant about engine curve]
To be fair for 2 and 3, the scrams jets are velocity limited and atm limited already. The rapiers kick into gear at mach 3 but the scrams are underpowered until mach 1.3 afterwhich they start scaling fast until Mach 4.5 then it starts slowing down again. In atm pressure it's max is only beween 0.03 to 0.3 atm anything above or below is less than 0.8 x Thrust x velocity multiplier.
Again, I'm just speculating based on how the numbers add up going backwards. If you wanted to reduce the power of the engines I'd probably change just the atmCurve key=1 0.6 to key=1 0.4 since that'd make the engines sit at 260kN. The problem then if you fly a super large plane and get it off the ground with only 260kN per engine... the engine is too strong at the top of the power curve for that craft. Currently, if you can fly a plane off the ground with 390kN of thrust (only a little more than a goliath btw at 360) the power curve is just right to get the craft into space with the scrams and vac engines and running a good profile. You can get really heavy loads into space too if you push the profile just right.
This is the J92 engine, which has a different profile than the J81 engine which is more OP imo.
The J81 engines run something like a jet/scram mixed. Anything below Mach 4 it runs like a powerful jet engine 550 x 4, then at Mach 4.5 it turns into a scram jet, eventually slowing down at mach 5.5. You don't see the performance tho because anything below 0.35 atm it gets a 50% nerf.
[/spoiler]
TL;DR the curve is ok but I won't cry if it's changed.
PS the mod at least loads in 1.2 prerelease, just there's no fuel in parts and 1.8.5 (not 1.8.5.1) has a few broken textures sure enough