Hadn't seen your reply fembear - so belated response, sorry.
The year 9 SAT, IMO & that of most English teachers I know, was a poor exam. At its best, you'd be assessing students on an extremely narrow range of evidence, whereas at GCSE you're genuinely looking at a substantial body of work completed over 2 years.
Any experienced teacher (myself included) got awfully good at spotting the 'tricks' you had to teach students to optimise their level.
None of them had much to do with functional skills (ie. actually using your own language, as a young adult, on a day to day basis), or GCSE English, or literature, or, well, anything but getting one's target grade on an English SAT.
& THEN they'd be sent off to be marked. Then re-marked, because (she says diplomatically) the calibre of markers was never high, & became ever more appalling.
Obviously, if it happened to be appalling in the student's favour, we'd bank that one!
By the time we'd finished, we'd have kids with the all-important level 5 whom we knew to be all but illiterate. & who would then be expected to get grade C at GCSE, poor sods.
I'm not saying all this because I fear being held to account btw - my SATs results were always good. The year before they were abolished, to universal rejoicing, my group got the best results in the school's history.
Am I particularly proud of that? Nope. The Head was on my back & I jumped those poor kids through hoops all year. It was miserable.
Why did we teach to the test? Because we didn't have faith in our teaching? No, because if we didn't, the kids didn't jump that hoop quite so neatly, the school didn't do as well in the league tables, the Head threw his toys out of the pram, we didn't get our performance related pay increases -& so one narrow & unreliable test KO'd a full year of vital education.
I'm not spouting theory. I've been in the classroom, full time, for 10 years. I wouldn't still be doing it if I didn't think I was genuinely doing a good job. In my professional opinion, my year 9 students are being far better taught now that they no longer have SATs.
GCSE isn't perfect by any means, & it never will be. I'm a bit sceptical about universal exams anyway. But it's a far better measure than the SATs.
To answer your point about 'high score' coursework, IME children rather like being told that their current effort is an E but they could make it a C if they did x,y & z. They like the clarity & the challenge.
If you think about a 2 year GCSE course, based perhaps on 40% coursework & 60% terminal exam, leaving coursework to year 11 is never going to be practical. When are we to cover the exam work?
We do the coursework early so that it can be re-visited, if necessary, in year 11 - but the main thrust in that year has to be the exams.