worth the jump to Cornwall IV's from Forte IV's?

Yes, but the Cornwalls are more full range than the LaScalas, right? No need for subwoofers. 

Yes, you are right. The CW IV is full range and the LS is not. But that horn loaded bass driver when mated to a good pair of subs is a thing of beauty.

I think the radiators on your speakers give you good low end response relative to the CW IVs. I haven’t heard the Fortes but I owned the CW IVs And they’re not bass machines. They will play loud and clear and with dynamics. They will not shake the walls unless you are really loud. They don’t go as low as less sensitive speakers. They just don’t. My current speakers have three 8” (92 dB) subs and they are noticeably better at reproducing bass (in the same system). 

Check your measurements again. The La Scala about the same width as a CW and  is only 10" deeper. It can also sit flush against the wall.

And as @ozzy62 pointed out, the sound is something to behold, subs or not (as in my case). My advice is to not sneak up on what you want; just plan and go for it. It will cost less in the end.

Keep the Fortes. An upgrade would be Volti.