Besides
initial purchase price,
fit, finish
and long term corrosion resistance
wouldn't the
design difference
between
4 into 1
and 'Tri-Y' 4 into 2 into 1
factor into value?

Mazda's new SkyActiv engine's press releases
make a big deal out of their new
4 into 2 into 1 header.

It is worth noting that the original MP Magnum V8 headers were Tri-Y

There have been tests where the Tri-Y design showed advantages:

http://www.circletrack.com/enginetech/ctrp_0806_exhaust_header_dyno_test/index.html

some designers really believe in that layout:

http://www.popularhotrodding.com/tech/0310phr_jack_burns_exhaust_manifold_header_tech/index.html

Is it worth paying 20% more in initial purchase price,
to get 2% higher torque?

How do you personally
trade off purchase price
versus
percent torque gain?

What should one pay for 1% more performance?

another vote for TTI