The way exhaust pulses are combined can have a very significant effect on the power of the engine. For the 2000 FireBlade (CBR929RR) Honda used a (cast Titanium) combiner valve that joined 1+4 and 2+3, or 1+2 and 3+4 under different conditions specifically because to maximise power under all conditions required the different merge styles. It was their first exhaust 'power valve'.
So the way the pulses are combined can make a big difference and the pulses from a 180° and a 360° V4 occur in a different pattern which Honda (and other exhaust engineers) have found require the different merging for each of the crank designs in order to maximise power. Let's face it, the way they merge Lefts and Rights on the 800 is much more complicated than Fronts and Rears, with more pipework required etc. They wouldn't do that if they didn't think it necessary.
Using the wrong merge style isn't going to make the bike unrideable, or even be particularly noticeable to some riders, but on a dyno it would be very noticeable and with the main thrust of this topic being to source a good high quality, high power replacement to the TBR system, going the other way to something that makes less power than stock is probably not what people here want.
This exhaust tuning should not be dismissed. When Honda were racing their multi cylinder 4 strokes back in the 60s, current thinking was that a separate pipe for each cylinder allowed freest gas flow and gave the most power. It has since been discovered that combining the pulses in the right way means one cylinder's pulse can help scavenge the exhaust of the other(s). This is one of the main reasons for the huge increase in specific power output of modern race engines.
I cannot help but think that anyone who tries to tell you that a Fronts and Rears merged system works well on a 180° crank 90° V4 is trying to sell such a system that they made because it was the cheapest way to do it and I'd need to see dyno comparison charts to convince me otherwise.