Even with your experience I'd recommend going with a tutor with experience of taking students though the exams - even if only for some mock exams. At least until you know what to expect.
I'm in a similar position in terms of my experience (and self taught). I decided I'd like to do some teaching and I thought the RGT qualifications would be good to have. I came in at 3, then did 5, 6 and 7. Grade 8 left - probably Easter if I get my finger out.
If you are experienced and can follow chord charts reasonably well then the "performance" and "bass pattern" parts of the exams should be easy marks. However as you progress, naturally these become harder, plus you will need to learn shed loads of scales and arpeggios / inversions in different positions and you'll need to learn to recognise chord voicings and intervals. Towards the higher grades music theory is tested a bit more - but not to the extent of dedicated theory exams.
Definitely help you musically, but I'd recommend learning to sight read alongside them (if you don't already) as they don't really develop that side too much. Also - they aren't cheap.
PM me if you want any more info. And good luck!