if you want to consider turbo swaps, b4 already has the best twin combination of the twin turbo series imho
btw this is all from memory so check yourself
VF33/32 - b4 turbos, best of flow and response.
V13/14 first gen 2 - too small
VF18/19 - the 19 is the biggest highest flowing, from the gen 2 gtb
VF27/28?? - haven't tried, but nothing special
Other things to try and getting the biggest cold-side wheel and the biggest flowing hot side turbo wheel and making a hybrid. Or (tried in nz) and one member on here wished he tried it - install a TD04 as the secondary
I used to have a ZeroSports
ecu - which is a good option if you can't decide on a custom tune, these are jap tuned but I never had an issue running U98 all the time
Also had a Zerosport Sequential controller which allows you to adjust the changeover point by controlling the two valves which pre-spool the secondary and then let it loose. It is the pre-spool point and length which creates the valley of death. you can find a sweet spot with this controller that means the changeover occurs in a 200rpm range. This was a revelation.
They are a joy to drive when working well, 14psi at 2000rpm down to 12psi on changeover and 18psi after 3800/4000 make them great at low revs.