We recently built a bolt on top half plenum for a VL turbo, and one noticeable difference was an improvement in throttle response. We did fit a slightly larger throttle body (61mm) , and I assume that could be largely responsible. The car was fitted with a front mount intercooler at the same time.
One might assume that with the fitment of a front mount, with all the associated piping and volume from the cooler that throttle response would suffer, but it definately wasn't the case.
We are working on a bolt on top half setup for a R33 GTS-T now, and I will let you know the results. If we have time, we will compare it to the stock plenum with a 120deg pipe fitted. One thing that concerns me with the stock centre feed plenums is the fact that the air blasts directly into #3 & #4 runners. Although this may not be important on a stock turbo car, you would assume that on a modified car with a larger turbo, etc. the increased air speed could turn this into a problem.