I'll try to get back to this before Tuesday, but I'm glad. I hope it works out really well for you.

One of the additional reasons for an oversize filter that I hadn't mentioned is the problem of VS pumps. Most of these pumps -- including the Whisperflo VS -- are actually extremely high volume 3HP pumps that can pretty much DESTROY most smaller filters if they are inadvertently run at full speed / full power! They ought to come with a key-locked maximum setting, based on the filter & piping they were matched to, so this couldn't accidentally happen.

The Pentair Superflo VS is apparently 'just' a 1.5 HP pump (PLENTY large enough for a 20K gal pool!), so the problem is reduced. Even so, that pump can potentially generate nearly 40 psi, if driven against a dirty filter that is undersized!