FMIC should have at least a cross section bigger than stock :
air go through 8" x 3" section on stock intercooler so don't got smaller than that.
We went from stock to 9" x 3.5" (with 3" inlet/outlet) on my friend delta and spool + top end were both improved.
I Run 12" x 3" x 18" (3" inlet/outlet) fmic on my 400+ coupe and I think it's nearly the best I can do for under 300€.
I would like to go to vertical flow FMIC to improve piping length and reduce pressure loss but price and routing will not be the same : at least 600€.
The small amount of reading I've done seems to point to vertical flow Fmic being used in high pressure turbo applications and no significant gains in lower pressure turbo set ups.
But I've only really started to look into them.
The vertical flow I'm talking about is just to solve the problem for short piping and having a big cross section area on ours coupe.
Of course if you can manage to do the same with horizontal intercooler then the result will be the same and there is no gain to go vertical.
I went from stock to 7"x2.5"x18" (2.5" inlet/outlet) -> more lag but lower IAT
then from 7"x2.5"x18" to 12"x3"x18 with 2.5" Inlet/outlet (in fact the external diameter is 63mm but internal is more about 55mm) -> less lag than previously
then from 12"x3"x18" with 2.5" inlet/outlet to 12"x3"x16" with 3" inlet/outlet -> less lag than previously
Even on stock turbo you will see good gains by changing for a big FMIC