This is something I've wondered about too. At the time, Crist's endorsement was treated as a big deal. IIRC, the McCain campaign was low on cash, as Romney could always outspend him, because he could fall back on his own fortune supplementing donations to his campaign. I remember reading a story years ago about local Florida Republican politicos taking Crist's endorsement as a sign that they should put Crist's turnout operation to work for McCain.
Is all of that really worth as much as the 5 point margin of victory McCain had though? Probably not. He probably would have won anyway, though it would have been closer. Note that the polls were seesawing back and forth between them in the week+ leading up to the primary:
https://uselectionatlas.org/POLLS/PRESIDENT/2008R/polls.php?fips=12
Another interesting scenario is if Rudy had not abandoned NH, Romney probably would have won it narrowly.
Of course both these scenarios fail to account for something major. Both Fred and Rudy essentially became stocking horses for McCain who they were friends with. Rudy and Romney were at each other's throats and so when he started to under perform and collapse nationally, he made his "Florida stand", which turned out to be a paper tiger, but it did lead to his support in NH migrating to McCain.
Meanwhile Fred went so far as to use time in a debate allotted to campaign commercials to attack both Romney and Huckabee on taxes.
And mutual hatred for Romney, led McCain's people to back Huckabee in the WV convention to deny Romney a needed boost into super Tuesday.