Lag for road racing can be cured by correct turbo sizing. In either case, there's always a compromise. I doubt you will need the power of a large turbo on a road course..if you will be dropping down to such RPM levels that you will be out of the spool range. You'll get just as much lag on a twin setup capable of equal power levels.
Regarding the twin making more power. Turbo Mag did a test a little while back, they dynoed a HKS twin kit back to back with a Greddy T78 IIRC. Pound for pound...the single kit matched the power of the twin...and lag was so indifferent..it was negligible.
The JUN AKIRA Supra uses twin T88's to make 1400hp. There are a few large single Supra's out there making 1200-1400rwhp! Just remember...with the addition of another turbo, you have to deal with the extra heat it makes, both on the exhaust side..and the compressor side. To make the massive amounts of power to exceed a single turbo, you would need a very large, very effecient intercooler system. Basically, the point that a twin turbo setup would surpass a good single turbo in power making ability is high enough that the amount of HP would be ridiculous. Why waste so much time and money, when you could do the same with a single?