Close to the TB. keep the inertial effects of the column of air working to your advantage. When the BOV is closer to the turbo all the air between the BOV and TB virtually comes to a stop. when you are back on it after a shift the turbo then has to re-accelerate all this mass of air to get it moving again. with BOV close to the TB all this air is already moving...so it is less work for the compressor.

we are talking very minor differences in response time...but if you are looking for response..then closer to the TB is better.