First, I apolgize if I am getting in on this thread too late and this is considered resurrecting.
I would say that they are too small (the 51mm ones). You are better off using 240SX or stanza throttles which are cheap, plentiful, and much bigger. Rememeber that the flow area = PI * r^2 - so when you increase your diameter by x, your crossectional area increases by (0.5x)^2! That square is killer... it makes the difference between
Two 51m throttle bodies = ~40.9 cm^2 areaOne stock 90mm TB = ~63.6 cm^2 area
So the potential flow, particularly under forced induction is much better (and with less heat). Granted, intake velocity might be improved in some ranges with the smaller throttles, but I think you should go to something like two 70-75mm for an upgrade that gives you the same flow cross-section, with better velocity than stock.
Here is a writeup on using Stanza/240SX throttles on the VG30DE/TT plenum. Warning, you may need to register.
http://www.twinturbo.net/net/v...31443