What is the difference between the Furman Elite 15PFI and the Elite 20PFI? They both share the same dimensions and weight.
I know the 15PFI is for 15 amp sockets and the 20PFI for the 20Amp socket. I examined the manual for both items there is no mention about not using the 15PFI in a 20 AMP socket.If they are the same why pay twice the price for the 20PFI. I asked these questions by e-mail to Furman twice, but never received a reply.
Current reserve (peak current) is 55A vs. 45A. Your amp takes current from the mains in form of current spikes that have much higher amplitude than average current. It is difficult to say if you need 55A or 45A reserve. I got 20PFI used (almost new) for about $600 to be ready for any amp I can get in the future (and also because it was very good deal) - otherwise I would get 15PFI (I don't listen very loud). AFAIK it does not have any perishable parts, so I bought it used and intend to keep it forever.

Power factor correction is most likely combination of large inductor and large capacitor that stores energy and can deliver up to 45/55A peak. It would be hard to estimate current crest factor (narrowness and amplitude of the spikes) because it depends on design of an amp. So, if you don't listen loud get 15A version otherwise 20A. With available current it is always - "the more the better".
I've also been looking at the Furman conditioners as an upgrade to my power conditioning at some point.

All other features aside, a power conditioner capable of delivering more current than currently needed will probably never hurt if the additional cost is digestible.

Peak current delivery capability of a conditioner seems to me to be one of the more justifiable features for spending more on a conditioner if needed. Like amps, conditioners capable of delivering more will be bigger, heavier and justifiably cost more accordingly. It really depends on the current needs of the power amp if used with that. USing for everything except current hungry power amps is a reasonable option as well I think to keep costs down.

You can't put out a fire if enough water is not delivered fast enough through the hose, no matter how pure that water might be. If the power conditioner cannot deliver the current/power needed, then I would chose to not use it with the power amp specifically.
Yup. I bought a Furman REF20i here a few years ago.
I like that it has balanced output.
$1,400. was a good deal especially as I could actually go pick it up with my car.
I like it a lot.
I alsi bought a PS Audio P-600 just for it's adjustable A/C frequency output.
I use it at 110hz for my digital stuff.