If you have already contacted a header builder then go with there recommendations. If ultimate performance is your goal then the primary size and length are very important, so saying you are using 2.125" to 2.5" for your primary diameter tells me that you have no idea what you need. Get on a site called headerdesign.com and input you HONEST engine and chassis variables and you will be shocked what you end up with in actual primary diameters and length. as for the collector, it is just as important and the primary's, sizing and length. Everything after the collector is just a performance inhibitor in one way or another so you should just focus on the header and collector configuration for now. You should not have any problem with power on the street with a potato in the muffler if you have 700HP in your street motor don't you think?
Bruce