The 15" is more than enough for 35's. I can lock mine easily. Anymore braking power would appear to be useless to me. I'd imagine the 17" kit would be too much brake for 35's.
Blaine has stated that the 17" BBK is meant for 40s or larger (or maybe it was 38s or larger). Either way, it's a mistake to have that 17" BBK on 35s (I made that mistake on my last one).
From everything I've read, the 15" BBK should be just fine for 35s.