I love A Level Art. I nearly didn't take it, but at the last minute I swapped Drama for Art. Like you, I was thinking "it's too much hard work! I'm not good enough, etc, etc".
I got a good grade at GCSE, but I don't think your grade will impact how easy you find A Level, as as long as you are explaining your ideas and developing them, you will get marks. Personally, I don't find Art too much work (it is BY FAR my easiest subject) - I'm in Year 13 and I finished my coursework back in October/November, so now I'm just creating whatever art I like in class to expand my portfolio, until February.
With the new A Levels, your course should be even easier than ours, because you have 2 years to do all your work - not try to squash 2 loads of coursework and exam work into both years!
I say go for it - if you don't like it, just swap. You'll probably change your mind about which subjects you're going to take 50 times between now and then, if you're anything like me, anyway.