I second that. I'm using the K&H O300D (predecessor of KH310) and a Genelec 7060B sub in a 50 square meter room and a listening distance of 3,7 m, and although the system is now 15 years old I'm still very happy with it. I do play loud sometimes but I have never seen the flashing light on the O300D which indicates too much SPL (clipping amps, actually). Shall the O310 break one day I'm going to replace them by KH310 (or whatever Neumann is selling then).The only differences between the KH420 and KH310 are the cleanerverticalhorizontal directivity (limits of the compact layout) and higher SPL capacity. Since the KH310 manages 100 dB at 1 m at 100 Hz for 1% THD alone, and you plan on using subs, I don't think you need the KH420.
Which is why I'd look into other gimmicks than SPL like coaxial and cardioid response if I were you (and had your money).
Adding the sub to the O300D did improve bass and mids significantly (subjective listening - let me add the Uh huh in advance). Since one of the major improvements of the KH310 over the O300D is 7 dB higher max output and reduced distortion in low bass it may be possible that adding a sub to a KH310 is not as necessary as with the O300D.
I too would recommend a sat/sub combo over a big speaker. Usually the best position for deep bass (room modes) is not the same as for best sound stage, and a good sub goes deeper than most big speakers. The sub creates a 4 way system which may sound cleaner in the low mids compared to a bigger 3 way system. And you can use the sats in a smaller room should need arise some day.