Personally, I find that art a-level is actually very similar to what I did at gcse but that might be because we were given a lot of freedom in my school. It is very time consuming and I end up in the art rooms a lot trying to finish a project in time but if you really liked taking art for gcse and get a good grade in it then it's worth it because you get so much more time to actually create something meaningful in the lessons. I also take 3 other subjects and sometimes it is difficult to get work done for them due to spending a lot of time on art course work to be honest- but again I think it just depends on how much you like taking art for it to be worth doing.
In terms of the designing career stuff- I guess it is down to what the college/6th form offers because mine doesn't offer any product design a level so the people who want to go in to architecture are taking art and doing a more focused EPQ as well. But I think taking art is more based on if you think you will put the time into it because otherwise you won't get a good grade.