cSe - I've spent plenty time on the 8'1 / 8'4 & 8'7 & like most performance surf SUPs, it's a board best ridden as small as possible so hopefully you get a chance to demo the 8'7 (just throwing it out there cause the stability is surprisingly good) / 8'10 & 9'1.
As you mention, your 8'10 Speeed has you covered in less than ideal conditions & your 8'7 Flow in average conditions so why not aim for a slightly challenging (only at 1st) Acid?
For us mere mortals but someone that also paddles a fair amount, my crude analysis is:
- for cleanish conditions, 110% volume vs your weight i.e. that puts you on the 8'7 (you would have your work cut but once you tame her, there's no looking back)
- for average conditions, 120% volume vs your weight i.e. that puts you on the 8'10 (the sensible option)
- for a cruisy ride, 130% volume vs your weight i.e. that puts you on the 9'1 (a safe option)
The 9'1 is going to be wider than your Flow (I personally feel you get more performance out of narrower boards) however it's not a shape where you have to worry about getting your back foot over the fins (it turns from anywhere & in more critical surf, you will land up getting your back foot over the fins otherwise

) + it's a shape which feels smaller than it really is.
PM me if you want more details otherwise good luck with your demo