As Menno and Mark already commented, the iS87 actually is a good "compromise" between too small (ie : suitable really for GPSSS only sailing) and too big (great for rec slalom but not so competitive when push comes to shove in GPSSS speed etc).
And right now in end of model "run out" mode against the 2008 season voards, iS87 could almsot be the performance "bargain: of the year. It will still do OK on the RS 7.2m in lighter slalom conditions, althouth not nearly as sweetspot as the iS96 in marginal 7.2m conditions (where the iS96 is really flying early, all the time, the iS87 has to be guided with a bit more skill/trim and is a little more vulnerable in lulls or mark rounding if the rider is heavy. But considering your described use, this really should not be a huge problem, especially when considering the iS87 proven GPSSS range as well !!)
The whole thing about recommending boards for GPSSS is that most often the guys focussed on this side of the sport (or selecting equipment for it) are not very "compromise" type guys (at least when it comes to discussing the "best" board for the job.) That's to say, they often don't care about the 100 great fun days they had on iS101(etc) when on one epic GPSSS day they all got beat by iS87 (etc). You get the idea. (Mark and Menno, that was not commented at you guys, I know you have a good balance on that ) But in the end it's all about making a balanced choice ("compromise" is a bad work to hardcore speeders ~ but that is what it really is !!). iS76 would be the hardcore choice but not enough compromise towards what you will be using the board for on those other 99 days..
Boards like PA ride very lively at speed and so they naturally feel fast/er than a calmer board at the same actual speed, so having an accurate GPS (etc) benchmark is important. Having sad that, PA86 (bigger brother to PA80) is definitely a fast board and although he never fronted to defend it, I once bet a guy a case of beer I could beat his speed board using my PA80.
One of the key factors with "speed" is really about expanding the window of performance(potential) as much as possible, to ensure the best chance of the best performance on the best day. With PA80/86 etc, a suprising performance may (!!) be possible if absolutely everything else is in "perfect" mode, however the reality is that this is a very difficult (or rare) scenario, and on most occassions having a board (or equipment) that widens the opportunity for optimum performance in given conditions is the choice most likely to deliver. If you had to tune up a PA86 for GPSSS, I would suggest a small A (US) box fin of freerace or raked slalom style, fast foil and size around 24cm, with a sail around 5.5m. If you REALLY wanted to be serious, fill in the spare fin box sot with a (removeable) poly filler etc. My bet was with wave fin (22), no tricks. But no doubt eiether the iS87 would have been an even safer "bet". And I'm not a gambler.
Cheers ~ Ian