( am partial to Progressive Dyanmics 9230 over the IOTA but to be frank.. Not much difference between them.
My recommendation is this: First post what converter you have.. That way we know if upgrading is even indicated
Second you said you are going from a pair of Group 24 to a pair of Group 27.. WHY? These are MARINE/deep cycle batteires.. You really should think about DEEP CYCLE batteries. GC-2 (Six volt golf car batteries). They are a bit taller than the Group 24's but should fit in the same tray.
Here are some numbers 20 hour Capacity figures (C/20)
Group 24. 70-75
Group 27, 95-100
Since these are starting batteries (MARINE) first and deep cycle second, you really should not use more than around 30 percent of the charge
Double the amp hours for two in parallel
Six volt GC-2's hold 210 to 230 (even as much as 250 but those get real tall) you double the VOLTAGE when you put 'em in series so they basically become a 4D.
They are DEEP CYCLE designed to go down to half full without major damage.