We have implemented tools to identify member accounts using insecure passwords and will be locking those accounts until their passwords are updated. Don't get locked out of your account - update your password today, and ensure your account has a valid email address on file. Read more here...
A single Walbro 450 in-tank might make it to 700 but .... I doubt it.
Keltalon needed a boost-a-pump putting 17.5 volts onto his Walbro 450, in order to make 700 hp and that was front wheel drive (GST).
Then he had trouble with that setup and ditched it.
Is the idea that you want a fuel system that will support a turbo bigger than the HX-40?
I would probably do whichever is easier really. Because you don't need 10AN on the feed, but I don't see anything wrong with putting 10AN in if it's easier, or just as easy. I would think that 8AN would be easier though. Easier to run, because it is smaller.
8AN for the return is nice and is plenty big. Fuel pump arrangements of this size, there are so many choices, none of them are 100% ideal in every way, there's's going to be a little pro and con with any pumps you choose. If you choose to go a little overboard with the pumps, the 8AN would still be plenty to handle the return flow, which could be a lot when the car is just sitting there running at idle (at low rail pressure).
People have done quite a lot with 6AN on both feed and return lines. So with 8AN on both, you would be what I would call "nicely robust" in that area.