I have not tried the new 2014 90/97 as yet. Hopefully I can try one soon
Last year I did test an older iS87 against the W58 and the W58 was faster on super flat water. I imagine that a W63/65 might be faster than a iS97 with an 8.6/9.5m (maybe)
No I not tried a 9.5m on the W58, but I have used an 8.6m several times on the W58 with 30/32 and 34 fins. Max speeds so far 37 knots (I think I can get 40k max one day). Normally this combo works from 15k winds but also needs bigger gusts on the speed course up to 20/25k to get that extra acceleration to reach higher speeds. This is when the W58 is much faster than Slalom boards. I have used my 9.5m on the iS107, quick but not like an XL speedboard once nicely overpowered.
This year in the UK, we have had a lot of very gusty winds, 10k gusting 20k. This is the typical conditions were the Swedish speed sailors have gone very fast on their CA63's with 8.6/9.5m both in Sweden and Kapathos. I figured that a bigger version of the W58 might catch up with the Swedish light wind speed specialist