For the record, I'm copying information I've found from those pushing more than us, such as the S63TU and N54 guys, so with that said...
NGK says one step for every 75hp and to err on the side of colder for safety reasons, so being at 150 over stock already, I'd suggest two steps colder and gaps around .20-.22 per results I've seen over on the S63TU and N54 boards when pushing the boost levels were running. I'm currently running NGK 97506's but I'm thinking about trying the 93206's that the N54 guys suggest for being over 700hp.
http://www.n54tech.com/forums/showthread.php?t=39190
Bolt ons are great and all, but tuning for each mod is the only way you're going to get ever HP for every mod done, so you're on the right path there.
I can't remember, but do you have JB4? Other than before and after dyno numbers to see if anything changes on your powerband, I'm guessing logging would be the only other way to see if the DV+'s are helpin.