I personally think that most manufacturers spend a lot of time building their engines and the crankshaft is one area where the manufacturers carefully balance every single crankshaft (the design at least) for a reason. You cannot tell if there is more vibration in an engine after modifying it b/c it already creates a lot of vibration, but adding to it makes the problem worse exponentially. Like the guys at Honda that create the S2000 say, the forces on an engine revving at 9000 rpm is twice that of an engine revving at 8000 rpm. We on the other hand are revving @30,000 rpm almost at a minimum for most engines, and so any excess vibration increases side load in the engine which rapes and pillages the engine of rpm which is one variable that affects horsepower.
I've seen people do this (me included), and have noticed no gain. I just want to say that there are good places to get power from, and then there are marginal and then there are bad places to look for more power. This turbo crank idea is usually a marginal one that most people should keep people looking elsewhere, like as you said, an increase in the % of nitro content in their fuel.
PS you all on 15% nitro, you really should at least try 20% for one gallon. I personally like 25-30% but, when you are using 15% and lower, you sometimes make it harder to tune an engine b/c the low nitro content increases internal engine temps. This makes it harder to get the right tune b/c you have to just sit pretty much until the engine reaches normal temp, if not, your engine will slowly begin to overheat, which is a big problem in the sport engines I've seen.
This is my (long) opinion, take what you want from it, but you have yourown opinion, so believe what you like.