Physically separating the tiers is more hurtful than helpful. If your web server and application are both sharing the same hardware, and your application needs more power but your web server doesn't, how will it hurt anything to simply run the combination on more servers? It won't. But it will cut down on the overhead of communicating with remote servers, which is very significant.
The only thing that matters is being able to run the CPU-bound section (or whatever the bottleneck is) on more hardware. The fact that the web server is now getting more power than it needs is not a problem.