For 700hp the brace is not needed and as said above I specificly wanted to test a non braced block in my own cars to see how far I could push them, keep in mind both of these (240z and FED) have been 2wd and auto) neither have had any issues at around twice the power your talking about.
For the 4wd guys and specificly the ones doing drag anywhere from 800-1000+ its a good idea, I have a few kits in VERY quick street GTRs (8 and 9 seconds in street trim) and they all swear by them and when I havn't built the bottom end myself, the customers engine builders also agree its a very worthwhile strengthening aid.
Be aware there are copies of my kit out there that I have been informed are being made from inferior materials, also, they are, as far as I am aware completely untested on an actual running engine and they are also more expensive, if you want PROVEN products, stick with the genuine RIPS kit.
Rob