Funny, i just read yesterday of someone who had a corbin made for his Tbird and just like i said, they have no understanding of the seat duct at corbin and they made it so that the bike was losing performance. The guy had to chop up his airbox to get it to breathe and gain back lost performance. But i'd bet he didn't get it all back. My experience messing with the intake is that it is really engineered for the best power given the state of tune of the bike. Things might be different if the bike was ported and had race cams etc etc.
I really don't understand companies like corbin. How can they look at the seat ducts and not get that they are there for a reason ! This to me shows a company who is all about the almighty dollar and not thier customers. And I HAVE seen a lot of people complain about them in the past so it's likely a company to stay away from. But i think as the seat duct goes, a lot of companies of this type would likely not take it into consideration. It's the way of the world today unfortunately. A company that DOES consider it would be one i'd have a lot of confidence it. If i were in that business, the second i saw the seat pan i'd have discussed the performance repercussions with the customer if he wanted a custom seat without the stock pan. I find it crazy that anyone wouldn't.