If your engine wasn't a dog on launch before and it is now it could be the advance, could be air fuel delivery as well. If it's not a case of slowly getting worse it may be the above or gearing. You'll want to check to see if the mechanical advance is even functioning, they can get gummed up and stop moving, springs can break, weights can jam. Same with the Vacuum advance, they can stick or the vacuum can can get a leak.
That curve kit you bought come with a selection of cams and weights as well as springs? Different shaped cams and different weight weights will give you a range of total mechanical advance as well as the speed it comes on. The springs will also vary when advance starts and stops in the rpm range. The idea is to find out what timing and advance curve is going to be best for your engine, An initial of 18 BTDC and a total of 30 at 2800 RPM may be great for one engine and crap for the next. Only way to tell is testing , by driving the car or on a chassis dyno. I set mine on an engine dyno, worked great with the engine on the dyno but I had to test and adjust the curve once the engine got in the car.
There are places one can send a distributor to along with info on the car, gearing, transmission, weight, engine specs, type of driving etc. and the company will set the advance curve using the supplied info. Even then I think further real life testing on the car on the road is the only way to get the timing and the advance correctly set.
