I truly couldn't care less about factory loads or not, my long range rig is actually built around a cartridge I designed, so 5.7x28's a cake walk compared to that. I'm in this for fun, and don't mind putting in the work, and 5.7x28's actually laughably easy as it uses one of the most common bullets available, pre-made brass, and has dies available for it off the shelf.
As for the 180B and PE90, lets just say I have more than a bit of experience with each, and neither comes close to being what I have in mind or I'd use them instead of my small cal bolts actions. Both are .223's, a HUGE step up from the 5.7x28, not even the same ballpark. Think LOUD and some recoil. The 5.7x28 sounds like a hot rimfire out of a 16" barrel (it will be even quieter out of a 20" rifle barrel, can't wait to hear for myself), and the recoil's near non-existant. Oh, and I turned down my opportunity for a P90 last year, it's not my cup of tea, what I'm after is just a nice, simple varminter of impeccable accuracy. Seems stranger to varmint hunt with a P90 than change over one of my CZ's personally.
And after all that, yep, I've tried the majority of factory small centerfires out there, have or have had rifles in .17 Rem, .22 Hornet, .218 Bee, .221 Fireball, .222 Rem, .223 Rem, .22-250, .220 Swift, and leaving out the wildcats. Now I want to try something new, even smaller, and gain something I lost and couldn't find once I went smaller than .221 Fireball; case geometry.
Yes, the CZ action's my favorite, and I have a couple micro action CZ's I've been toying with switching over. Time will tell, it would make an amazing gopher/crow cartridge, unbelievably efficient, recoil you don't notice, and 2,700fps out of a rifle barrel with 40gr. In a heavy barrel you could shoot come long strings before you have to cool that puppy down.
Hey man, whatever floats your boat. There's only 20 or so other .22 centerfire varmint or target cartridges out there already. Many of them, like the .22 Hornet, have the advantage of being readily available in the form of loaded ammunition, brass, and factory chambered rifles.
But like I said, if you must have the 5.7, you must have it.
Why not get it chambered in an M1 carbine action as a ranch rifle? It occurs to me that the CZ 453 American in .22 Hornet would also be also be an excellent little sporter to convert.
But frankly, why bother? AR180b's and SIG black specials work plenty good as varmint guns. IIRC there are non-restricted P90's floating around out there that you could use. I would if I could find one.