If you've used the first outfit for the last two years with no problems why would they try and pull a fast one on a valued customer? Don't forget, compression test results can vary depending on things like engine temp etc, if you look on the Lycoming and TCM websites there is some good gen on engines and low compression is one of the topics discussed. If your a/c has a PT CofA then the engine will need replacing soon anyway, perhaps they were suggesting that rather than start changing barrels this late in the engines life it would be better to replace it now. I can't believe that you would knowingly be misled, however, if someone told me I needed a new engine for my car I certainly would get a second opinion (unless it was obviously shagged).