When I went to -5's I had HKS dumps and Just Jap's JJR stainless front pipe installed. JJR pipe sizes are twin 60mm inlet, 76mm exit and best of all it was only 300 bucks. Works perfectly on my car no boost issues always rock solid 19psi. There didnt seem to be any lag or flow issues as the car makes more power everywhere when comparing the stock turbos/dump/front pipe dyno run to the -5 setup with the bigger front exhaust and dumps installed. Car runs a decent speed (117mph) at the 1/4 mile that supports the dyno figures and suggests theres no lack of top end power (400hp at all fours).
In 4th gear the boost ramp rate is almost identical from the completely stock turbo, dump and front pipe, to the -5's, HKS dumps, JJR front pipe. So the way I see it, the -5's should be laggy by themselves, and if the front exhaust was inefficient, then itwould be laggier still. I guess the rest of the setup with cam gears, tune etc brings the reponse back close to standard GTR levels. So for some reason the HKS dumps seem to work on my car. Standard dumps may have worked the same, who knows, but while I had the turbos changed I wasnt going to wait to find out they were a restriction, and then have to change them later.