HFCs with standard exhaust will just give you a slightly louder, rought/raspier kind of noise. It wont really sing like one with an aftermarket exhuast and HFCs, nor will it give you any more power.
In the manifold > cats > y-pipe > exhaust chain its the Y-pipe w/secondary cats that strangle the system, not the cats. Once you have removed that (with an aftermarket Y-pipe), then the next restriction is the cats, but not by much.
So in terms of noise, you will get a little, but not as nice (IMO) as an aftermarket exhaust, nor will you get any extra power. However if you are looking to get an exhaust and remap later then they will be worthwhile to get them now if thats what you want