If you're retaining the ST115 and want a new hi wind slalom board to push GPSSS performance then without question either the iS87 from 2007 or the new 2008 iS76.
(Yes, these will overlap on your PA86 in volume but not in usuage in terms of sailing style/conditions - and if GPSSS in more wind than ST115 is your thing, then taking a slighlty "smaller" than typical next board is also a good move.)
I'm only 92 kg and for me the iS76 is still a big GPSSS board.
(it's also clearly proven to 40+ kts - even with your front foot stuck outside the strap - as I found out on my very first run on the production 76).
For GPSSS speed obviously tuning still varies with conditions but for GPSSS "sweetspot" on boards like iS76 and iS87 think about sails in the large 5m (5.5m+) to around 6.5m range. Fins will vary accordingly but for downwind GPSSS think about 28-30cm (for your weight and these sails), or maybe slightly less with a more raked fin. The use of "slalom" boards (especially ultra modern wider tail designs like iS) for speed general requires (and benefits from) the use of a little more fin (extra lift for extra board) in the same conditions where a smaller speedboard (45-50cm) would be tuned with a smaller fin.
Cheers ~ Ian