I mainly use Naish boards(not sponsored) i have two SP boards(SP 60 and 95) and one Grand Prix board.
I love the Naish Slalom boards and i dont understand why more people dont use them, they a great boards. Before i started using the naish boards i was using starboard and personally i like the Naish boards a lot more, its a pitty that they stopped making their full on slalom boards.
I have the Grand Prix in the 135 only because i couldnt get hold of the SP in that size. The Grand Prix is also a great board and goes really well and i reckon it would be pretty similar to the SP135. I havent had the board long and have done 31+ on the board so far and i reckon it can do more.
I think fin choice makes a massive difference, for example the 135 came with a 48cm Naish fin but i've been using my KA Race 7.1 on the board with a C3 venom 40cm and the board with this size sail is really nice with this size fin when powered up, underpowered(just on the plane) it can feel a little small and you need to be light with the back foot to avoid spinning out. I havent used the 48cm yet, but I used a 44cm Naish fin in the board and it certainly didnt feel as nice and quick as when i had the c3 fin in the board.
The Naish fins that come with the boards arent bad but certainly not as good as using a specialized fine, i use C3's(not sponsored) in all my boards and they certainly make the board feel a lot better and quicker than using the stock fins.
One of my friends has the SP135 and LOVES it

he may comment on this thread..