Honestly there isn't any real science out there on this. (at least none that I could find) Back when Downey was in business, they had an article on their site about tri-y vs 4 into 1 that they conducted when they were trying to decide what to offer. Unfortunately I can't find it rattling around the net anymore but they concluded that there was negligable difference. I don't remember their reasons or if it was really even very good research, but it was all I've ever really seen. They ended up making a 4-1 IIRC as does LCE. Thorley makes the tri-y as does Northwest Offroad (if you're willing to roll the customer service dice with them).

I ended up with LCE full exhaust and definately noticed a difference in the butt dyno but no real proof to back anything up. I was going to go with Downey but just as I was getting near to pulling the trigger, it looked like they were going out of business, so I didn't want to get caught up in that. Long story short, my 4-1 header seems to give me an improved feel and throttle response at the very least. Beyond that I've never had it tested or anything to provide any real data.

One last option that could be a cheap thing to try out that has been kicked around here on YT.

Stick with the OEM manifold which is already starting to look like a tri-y. Add the downpipe/y-pipe from a 22r rather than 22re (pic to follow) and look how much more the stock exhaust looks like a header. Then at cat back convert to 2.25 inch sizes.

Here is a schematic of the exhaust parts:
http://www.villagetoyotaparts.com/im...ze/MA7063D.jpg
Note the differences in part #17410 depending on which engine it is connecting to. The part for a 22r is muh more headerlike. Just an idea.