Hi Happyhd
You asked... so here we go!

I have been working on this, but decided I need more info.
To insure best advice, I really need to know what you like/don't like about your custom.
How stability compares to performance etc.
and what you want from your next board.
Your ordered your custom FLOW a lot wider than stock.
Your volume puts your "weight to volume" ratio at a very floaty 1.62
Was that because you place a premium value stability?
Is your FLOW easy or just about right?
You seem to want to ride a shorter length.
Is that strictly for performance?
I think your X-wide FLOW would really neutralize any benefits of a shorter board.
I want to make sure that you end up with the performance and stability that will make you very happy!
You threw me a bit with the mention of the 8'5 SPEEED.....because it's only 27.5" wide and only 112 liters
That is going to be way less stable than your custom FLOW.
Please give me any info that will help me give the best advice!!
General thoughts on the different design comforts... and how they compare (for me personally):
The FLOW is wide mid to tail, so it is a very stable board in general.
I can ride my 8'10 at 107 kg + winter gear
The ACID is very thin in nose and tail.
While it is reasonably wide in the middle, it needs good skills if you want to go short.
I can't ride my 9'4 in winter gear
The CREEK was my answer to ACID type performance, in full winter gear.
While not a "stability" board, it has a lot of "stability tools" built in to it that make it reasonably stable for a performance board.
I widened the nose and tail and added some foam, to keep them from dipping under as fast as the ACID.
I easily ride my 9'4 in full winter gear
Thoughts on performance (for me personally):
Going short doesn't always mean performance
My 9'4 ACID is much higher performance than my 8'10 FLOW
My 9'4 CREEK is much higher performance than my 8'10 FLOW
I put all the dims together so you can see how they compare side by side:
I put your numbers (in red )next to the CREEK and ACID sizes that would be similar stability to your custom FLOW
I didn't use volume as the important factor.... I used width, because you could go a lot less volume and be fine (IMO)
These are not recommendations.... they simply indicate boards with similar stability to your current FLOW
By numbers alone, you should be ok on:
8'4 to 8'7 CREEK 1.4 to 1.53 w/v
8'7 to 8'10 ACID 1.41 to 1.54 w/v
This is where your answers on stability of your custom FLOW come in.