I have advised many times, that a Power Plate is the first add on you should install to correct the problem that exsists all turbo buicks that of uneven air flow to your cylinders. This is one of the best and basic mods that you should do.... Turbo TC
I run bfg drag radials p-275/60r-15 on my gn I have rolled the inner lip took about 20 min. a side they dont rub even with no air in bags and two in the back seat. they ride well but have not got been in the rain yet. Turbo TC
We tested a hemco 70mm, the free flow figures were 58%diffrence in the flow from the richest to the leanest cylinders ,which is a lot better than stock or other plenums but not a good as a precision plenum and power plate.. Turbo TC
If you are running champion or ported heads with 55 or bigger injectors with a 63 or bigger turbo and big cam then I would say buy a precision plemun and power plate for that plenum for best results. Turbo TC:)