The most you are ever going to see out of a Cobra 29 with a RFX75 is about 55-60 watts, you have to first understand "what" the RFX75 is. It is not an "amplifier" is a "final replacement" (with too damn many wires in the first place) Basically what the RFX75 unit does is take your stock finals out of line and replaces the stock finals with the mosfet finals that are in the RFX75 unit (ie: ERF2030 / IRF520 upgrade).
The RFX75 is a great gimmick and it makes someone rich, but, it is designed for 10 Meter and HAM units that are designed already designed to accept the RFX75 add on board.
If you think for a second that you'll be seeing watts anywhere near 100-120, you're up to getting feelings hurt. If you want to see actual watts in the 100-150 watt range, do the RM Italy "stinger" boards, which are actual amplifiers, that are driven by the radios stock finals.
If you want to use the RFX75 idea, then just do a MosFet final upgrade and be happy with what you get, then go buy an external amplifier and do it without all the heads you get from something that will never do what you want anyway.
I'm a tech with about 30 years of experience and I just hate to see guys get their hard earned money stolen from them with gadgets like this, but, hey, if you like it, then I too could tell you it's doing 100 watts and take your money, but, I'm not that kind of fella.
Good Luck!