I'd be interested to know the answer to the second part of the question - "is the SL55/TTM/looped system good for low rpms as well as high".
In my business we call this "turndown" - the system might be good for your 550-600cc/inj/min WOT needs but what about your 'round town 130 needs at idle - is it rich or show poor fuel consumption (...as if that matters when you're in this zone)?

My calcs suggest you need 576 at 6000rpmm, 100% duty, 21psi boost but at only12AFR - something larger may be "safer" and more future proof....but by all reports Steve and Grip Grip's outcomes with the pump/injectors/rail were bang on with their stacked 22psi setups and Steve logs the crap out of every detail.

I've read there are Bosch 750's out there which could possibly put you in a sweeter duty spot (target 80%) and allow your tuner to hit 10AFR's (690cc - if he needs to) - but again - I'm not aware of the turn down capabilities and needless to say - Steve know's his $hit and would have explored all options...

Not to mention, the gamut of cooling issues you run into over 20psi....

PS - I got a quote from TTM in Sept at $420+post
https://www.crossfireforum.org/forum/crossfire-srt6/53768-stacked-pulleys-23-psi-no-codes-3.html#post645398