a) Excellent results! Always good to discover more mileage than you thought possible from your system.
b) Why butcher the LNB and feedhorn? The Inverto KaKu comes with an integrated Ka-tuned feedhorn than matches the CM120 quite nicely. Don't get me wrong - you should of course, as you say, just go ahead and do it. But I doubt you will get more oomph from the CM120 Ku band feedhorn. But again - it shall be very interetsting to find out!
.... I would agree with st1 & scopus on this one recalling my past dealings with modding Ka feeds and it could end up in tears and a knackered CM feed.
If you just can't stop the urge to have a go then you must, i know how strong these urges get lol.
I personally think you would be better off leaving the Inverto Ka/Ku feed intact as is because i think it's pretty good but give a lot more attention to fine tuning the Ka performance of the dish installation for absolute best feed focal point position to give maximum S/N figure, which will include fine tuning for skew as well and accurate bore sighting of the feed to the dish face. The Ku performance will not be a million miles off after that.
I still think you would benefit greatly by having the inclined jack fitted to the 1224, you then have total control over the dish pointing for accuracy.
We are not a million miles apart in the country, i'm only up the road nr Mansfield so there should not be too much difference in received signal strength between us --- famous last words as Ka reception can be very strange but iv'e just looked at the two frequencies you listed and the S/N levels you had , i tweeked my 1.2 Gibby / Inverto Ka up for maximum smoke using elevation + E/W nudging and my levels are
like this: 21537 H--you 11.1dB, mine after tweeking-14.5dB. 21540 H--you 8.7dB, mine after tweeking 12.2dB .
Your def hooked lol.