That's only partially true. Twin turbos usually make more power at lower engine speeds, while single turbos will make more top end power while compromising low end power. Lag is really not an issue to debate; it usually comes up re: twin turbos, but can be remedied by running them non-sequentially.

Performance-wise, both can be great street and track-driven setups. It just depends on each particular vehicle and driving demands. You don't see many cars straying from factory designs, except for Supras; which it is very common to see a TT converted to single turbo. Why so common? Probably because somebody made the conversion possible, made it marketable, and is continuing to bank off of it. You can build equally great cars from either setup.