G'Day Don.
Welcome to IIS!
I don't have any suggestions re: the UHC/LPR filter. I'm fortunate enough to not have a lot of need for one so I don't have one.
It's difficult to know if you're expecting too much or not in terms of sharpness, but I notice you made no mention of collimation.
If your scope is out of collimation, then your images won't be as sharp as they could be. You can check you collimation by focussing on a star in the centre of your FOV, and then throw it out of focus. If the defocussed star image becomes a disc of concentric rings then collimation is good.
If the defocussed star image has the centre of the rings eccentric, then the collimation is not so good, and if the defocussed star image is not circular than it's really bad!
The collimation on SCT scopes is done with 3 screws under the secondary mirror cover. I cannot recommend Bob's Knobs enough to replace the factory fitted screws and make collimation so much easier.
Once your collimation is good, do a search on IIS for "Bahtinov mask". There's an excellent thread there on how to make one to help you to focus accurately.
Other than that, "seeing" can play havoc with the clarity of your views, especially at high power. Being in a relatively built up area, you may find there are sinificant thermals rising off surrounding houses during the colder months as everyone cranks up their heaters, and this will contribute to poor seeing.
Al.