I can do small areas to a good standard, longer walls I find it tough to get it on quick enough, especially on a hot dry day. I have found a sponge float very useful on bigger areas, can help rescue the finish if you didn't get back to it quite in time. I dont like doing it, but it's often tough...
Have put those panels up you linked to over old tiles, stuck back any loose ones first, it works but don't like the panels myself. I might try the composite click tiles next time I need to go over some old ones, only 4mm thick so will work in most cases.
If your existing tiling is well stuck, you can tile over the top of them, if there is space between the existing ones and the taps and it won't take up all the rim edge of the bath. Taking the old ones off, especially if they are on plasterboard, can leave you with a wall full of holes. If you...