a .65 bass string for the low B. It works extremely well.
I've been thinking about doing this since I got mine! It seems like the regular-gauge B string doesn't put out as much volume as the rest of the set. Adjusting the pickup pole-pieces helped, but with a 25.5" scale I bet a bass string would sound really tight, especially since I dropped the B down to an A.
(god I'm a nerd)
The Gauge on the low B has always been total science, really. You have to find the right balance.
For instance; if it's to thick, it'll sound all woofy and muddy. If you have it too thin, it'll buzz and flop like there's no tomorrow.
First of all though, and I believe you've heard this a squillion times before, change the pickups!
Stock pickups in recent day Ibanez models have been known to be less than impressive, although with sufficient tweaking of the rest of your gear you can make it sound cool.
I for one, managed to squeeze a fairly decent out of an RG7321 i used to own.
I used a POD Xt, directly into the computer, and I nailed a pretty cool Meshuggah "Nothing"-style tone.
This can be heard, here:
http://download.yousendit.com/D472737A432BD329The guitar was tuned down 1 1/2 step for that clip, by using a regular set of .12's, plus the .65
I had bundles of fun with that tuning

That said, I would think that a .60 low B would be the absolute minimum, for dropped A. I don't quite know how the string tension feels on an RG1527, it's fairly loose on S7420, which is why I upped the gauges, but it was a little tighter on the RG7321. I don't know if it was the fixed bridge or what.