Yeah single is heaps better for performance, tuning, lag etc etc etc, and has been discussed many times.
But.. I'd like to pipe in with my 2 cents. Especially as you have an V-spec. If you do opt to go single, keep all of the factory parts and matching engine if you do replace it as I can confidently say as these things become more and more rare in factory form and the price will only continue to go up.You'd be mad to cut up and change another Vspec to the point where it can't go back to stock if you are an enthusiast, that said end of the day it is your car, your choice.
Imagine all the XU1, Coupe Monaro or Phase 3 owners that chopped up good standard examples only to have them worth a fraction of what the OEM ones ended up as these days. Rx7 Gen 1's are another example.
If you really wanted a big single monster, surely you could sell on the vspec to a collector and get one already built and put some money in your pocket for more mods!