When I upgraded my phono stage from the one built into the CAT SL-1 Mk. III preamp, I went with the Lamm LP2. The Lamm phono stage is definitely superior to the CAT as is the Manley Steelhead, which I ran in my reference system for several years subsequent to the Lamm LP2. Both the Lamm and the Manley phono stages retail for prices in the $7k range.
I can't comment personally on how other phono stages at lower price points might compare with the CAT other than to say that the phono stage built into the CAT is excellent. You will definitely have to spend some money to improve on it.