The 20~25% gain would be about max.  I would figure closer to 10% average.

A couple things will work in your favor.
You won't go 5 days with "No Sun".  It may not be bright, but you will get some charging everyday.
MPPT will put more amps into a lower battery than a fuller (is that a word?) battery, given the same 'sun'.

The 650AH seems reasonable to me.  And that is 3 pairs of 6V golf cart batteries from Sam's Club / Costco, though I would probably go with 4 pairs.
Might look into 2 pairs of L16 or L16-HC if the wallet can handle it.  Interstate is less expensive than Trojan.
It will make a lot more power in summer than the 3 hour insolation.