The only compromise he would need to make to win the GOP nomination would be on foreign policy. That's where republicans who like him but wouldn't vote for him part ways with him.
Which he would never do. If you have any doubts (and don't want to read all his writings for the base several decades), listen to the first Republican debates of the 2008 primaries. Paul is by far the most consistent opponent of the corporate war machine currently running for office. He'd make Obama look like the corporatist warmonger he is and it's this trait that is his biggest 'selling point' as a potential Republican nominee.
But I agree with you that this is where most mainstream Republicans would withhold support for Paul. It's my hope that, this time around, Paul rides his primary run to a peak (right up to the point where the Republican power players decide to sit on him) and makes a break from the party. He'd still likely not win as a Libertarian or independent candidate, but he could just give momentum to a new alternative to the Democrat/Republican same old.