The LFB and KS-25 cluster aren't very OP when used with the large parts. In fact, their real life equivalents are much more powerful and weigh about the same (the SSME only weighs 3.5 tons and produces more than 2200 in of thrust). They're only overpowered if you use them in a an asparagus configuration, which is true for pretty much every engine. The KR-2L, on the other hand is OP. It's meant to be an upper stage engine, but it has a high TWR than the KS-25 cluster and is also more efficient. I'm finding that my upper stages are larger than my first stages!