Custom is mostly not necessary...unless you're just doing it to support an artisan because you want to. Otherwise, I can solve 99% of the "fit" issues that people have with an appropriate sized bike and a stem swap.

A new bike? My average speeds don't vary dramatically between my CX bike and my road bike. The variance is enough that were I racing, say, a 100 mile stage in the TdF on my road bike I would beat me on my CX bike by several minutes. That's HUGE for a professional racer. For my regular riding it means squat. I just ride the road bike for fun, and because I like it...but I'm realistic about what it does for my performance.