Twins will initially spool slower as the secondary (spools first) has a bigger restriction to the intake than it did as a single. As soon as you get a little bit of flow (maybe more depending on turbo size) that goes away and your spool increases.

A big turbo needs fuel to cool, so at lower boost it doesn't move as much air and at too low of boost it's not efficient yet.. so you can have too little of a load when towing to keep the turbo moving the air you need and run hotter at 250 hp than you did at 600 hp.

If you want to be able to play with 600 rwhp, but still tow at a cost lower than twins..

Modified Stage 3 from Turbo Resource, Cam, Water Meth, 50's and a Smarty...

 

For a turbo I would look at a ED(Engineered Diesel) 63-68-.88 I heard really good things about it but it is pricey. I'm pretty sure they have pushed up to 750hp.

 

Due to just upgrading my cam I am changing my opinion. You can do this and get good fuel mileage at the same time.

Turbo:
HTT 62/71/13ss or an ED 62/68/12or14

Cam upgrade:
Hamilton 178/208

Injectors:
F1 Flux 1.4 or 1.5

If you are going to push well over 40psi of boost you will need upgraded valve
springs.

 

Spooler, what valve springs would you recommend? I am trying to achieve similar power as OP.