Am currently using scart to TV. It is a panny X10 plasma (740ishp).Great SD performance I think.
If I replace it with a full 1080p capable TV, and still use the scart would I see an obvious drop in picture sharpness/quality? ( change to a Panny ST30 plasma)
Actually there is... there are more pixels on a 1080 panel and so more scaling is required. How well this is done rather depends on the make and model of TV...
Panasonic do have a good reputation here, if your staying at the same screen size you should be fine. If your going bigger but keeping the same viewing distance you may well find SD is softer.
Best thing to do is to get to an independant dealer and have at a look at the TV playing SD at your viewing distance and see what you think... oh and head over to the TV section of the forum for more TV related info...