First i want to say im running the Redemptive rEVOlution rom and i dont know if this will work on roms with the incredible framework, i think it might....
I installed a zip from Calkulin over on the xda evo forum for a compression fix for mms. If the pic is not over the size limit, it wont be compressed anymore. I have just sent about ten pics in a row ranging from 550kb up to 1.1mb and they all went through without compressing! I take no credit for the mod. All credit goes to calkulin over at xda.

I found it very annoying that every pic would get compressed. Glad to find a fix for that. BIG. THANKS TO CALKULIN FOR THIS.

Now when it does compress a message thats over the size limit, its still" over" compressing it. My limit is 1.2m and if the file is 1.3, it compress' to 44kb...What the hell? Just have to find out how to fix the over compression issue and im happier then a pig in shit!

link: http://forum.xda-developers.com/showthread.php?t=785271&highlight=5mb+mms+mod

Now as far as the 5mb mms size mod, I dont reccomend it because most phones can't recieve messages that size.