I am a Shoebox user as well, and I added a 9L Acecare tank to my "system" last year and really like it. I did not get the Acecare valve - I went with the Saber Tactical valve and their adaptor, as I wanted something more robust on mine. I basically use the 9L as an "air bank" and use that to refill my smaller Joe Brancato Guppy tank (just under 2L) that was my principal tank before the 9L came along. The Guppy goes up about 500 psi with every 100 psi that the 9L drops, which works better for me now with the high pressure fills that my Huben K1 and Sidewinder take - I typically fill them to 4000 psi, and I only got a few full fills off the Guppy before. Now I can have the Guppy full as a traveler, or fill the high pressure guns directly from the 9L at home.
It is important to understand that the run time of your compressor is based on how much you shoot, not how big the tank is - at least once it is full (it took about 13 hours of total run time to fill the big 9L the first time). I don't run the Shoebox any more total time than I did before I got the 9L tank, but I tend to run it less often for a longer run time now. It puts out the same amount of air in the same amount of time as before, and that air usage is all dependent on the shooting.
One thing I did along time ago was to add a calculation to my tuning files in Excel that, in addition to the normal air efficiency metrics, also tells me how that tune performs in terms or pellets shot per minute of runtime of the Shoebox. Most of my .22s run abut 6-8 "PPM" or so of run time, and that does not change as I change which tank I fill from . . . .
Go ahead and get another compressor if you want, but it won't change much in the big scheme of things. In my case, I bought a second used Shoebox off the classifieds to have as a backup in case mine dies, as I already have the parts and knowledge to maintain it and I really do like them. I should be all set now, but worst case I'll simply buy a CS4 if needed - or whatever the best going compressor like it is available when needed, which most likely won't be for many years (or decades). I think it would make sense to use the Shoebox till it dies rather than to keep it as the backup to a CS4, as most mechanical things don't really like to stored long term and may not work right when called up in several years. With that in mind, my backup Sheobox gets used every few months to keep it fresh. If you go the route you are planning, I'd suggest doing the same to keep it working well.
