Awhile back I made the 2.5 Super Y, a few here have them, it was more of an experiment than anything (even did a 2.5" into 3"Y once), at the end of the day most guys wouldn't see enough of a difference to bother, a single 2.5" hose run and tank plumbing gives a lot of return on investment with reasonable hassle, I call it the sweet spot.
As for 4 to the door, we mostly have guys try that out if they already have twin vac ports on their system, since that will tell them how much "kick" they will get from the upgrade and many already have a 2" Y splitter in the garage somewhere.
Replacing the plastic tank fittings can do a lot for #4 and #5 blower guys. Same reason we make the "Vanifold" for #6 blower guys, it connects three 2" hoses to two 2.5" hoses, (or even ALL 2.5 on all 5 ports) makes a big difference on older
Vortex/
Aerotech machines, as the current ones come with 2.5 standard these days.