Bah - is diving dry an option at all?
IMO, you simply cannot be too warm in a wetsuit, especially over multiple dives in a day. I do half of my diving in 75+ waters, and always look for the thickest wetsuit I can find; I always wish I was warmer, even in a 5mm; and half the people diving with me do so in a short-sleeve rash guard and board shorts.
But a 7+mm suit really has its disadvantages as well, and if you really get that cold, it might definitely be worth the extra money to spring for a tropical drysuit or something like a Whites Fusion ($1100). You may not even have to get any drysuit undergarments (and definitely not the $3-400 400g stuff).
It may be a nice alternative to spending up to half of that or more for a custom or altered wetsuit...