Honestly, from your description I would put the cat boarding off this season and spend most of your time this season riding off piste. A groomed black diamond really does not compare well to off piste riding in general. In fact, it is quite often terrain that is too mellow that presents the greatest difficulty to the inexperienced off piste rider.
Spend this season riding off piste in the nastiest, iciest, shittyest conditions mother nature throws at you and do it on all terrain including double black off piste. In addition, get some practice in riding deep off piste powder. Half of the battle with powder like this is learning to judge your terrain and where you can and cannot ride. All too often the innexperienced rider will drop a juicy looking slope through the trees without considering the quarter mile flat runout or the deep creek to cross at the bottom. Getting good at off piste powder riding takes experience and a huge part of that is learning proper route finding skills for the snow conditions.
You don't have to be stylish or throwing down badass tricks. It's even okay to fall; just be able to confidently ride anything reasonably well. When you can do this, you are going to enjoy your cat experience much more and really get more of your money's worth.
If I didn't live in Toronto I'd be with you on this plan to get better - I can certainly get lots of nasty shitty icy conditions locally but there's not much if anything in the way of off piste available to me, and deep powder aint gonna happen.
I will get to whistler twice this winter, one long weekend with the guys and a one week vacation, by the time you add up the cost of flying a family of four out there, accomodation, lift tickets, and so on the catboarding is a small expense in whole package so if I don't get 100 per cent out of it that's ok, I just don't want to be hated by 11 other people.
I see that at red mountain they have three different levels so it's easier to get in with those of the same ability, so maybe I'll go there next year, but this year no luck, unless there are 11 others like me who want to get together and book a full cat.
Maybe I'll try to get someone at the resort to go out for a couple runs and advise me if I should go for it or wait another season (or go to red mountain).