No. You change gear at 6500/6700 rpm and you are right in the power band again. The utter myth that the .63 housing is mega laggy comes from people who had a T3 and then stuck a t34.63 on and expected the same boost thresh hold as the T3 and ultimately get themselves confused with lag and boost thresh hold. Of course there is a slight difference but ive had the t34.48 then went to a 55 and then to a 63. Wish i had just bought my t34 with the .63 housing in the 1st place as its the best of the bunch. As for max power, 2.5/2.6 bar peak on a ported head and a decent cam will get you over 400BHP. I cant be arsed to lift the head on mine and get it ported but a BD14 inlet cam dialled in correctly works well as per my sig.