I don't think it's impossible to go to an unequal TS setup, I wouldn't even say it's really expensive either. The other option getting rid of the TS system all together, as it gives you maybe 300rpm of less boost threshold.
If you go down the modified TS route the problem is that as the pulses would be joining for the first time in the dump, rather than the uppipe, so it not ever sound the same, but could be close. If it were me I'd be retuning it afterwards, as I'd also do the post turbo exhaust at the same time, so you'd end up with better performance, rather than less, but only if you do the lot, and retune.
You would do it by modifying a set of Kando 4>1 headers and mix the pairs the wrong (less efficient) way, so you had the left hand bank and the right hand bank going through each side of the scroll of the turbo. This is technically less efficient due to the firing order as you would get two pulses from one side, then two from the other.
Swapping an entire header setup from a WRX, turbo (dunno bout sump) and getting a TCP 07 lib exhaust would work also.