Ehh... They're kind of OP if you're playing in sandbox mode. I tested the Rhino with an SSTO of mine and had to totally redo the flight plan for reaching orbit because the aircraft was going to fast and started overheating from aerodynamic drag.
That being said, the cost of these engines (for career at least) likely makes up for their ridiculous stats: The SSTO I tested the Rhino with costs 2,078,033; 2,000,000 of that is the two Rhino engines.
A few tweak suggestions:
Drop the price on the Rhino to between 15,000 and 20,000 (The current cost of the Rhino would give even the US military second thoughts) (actually it probably wouldn't)
Increase the thrust in the Bear's HighAtmosphere mode to around 250 (The difference between the Bear and the Mouse when both are in HighAtmosphere mode is currently negligible)
Other than those minor things, the mod is awesome! Will definitely use it extensively on future SSTOs and Hypersonic Spacecraft.