Hi Patrick, as you suggested most people probably will agree that a lake B+J setup for a dedicated waveboard is perhaps a bit odd. Maybe that's partly why you haven't been rushed off your feet with replies?? Anyway, as I've some experience with larger Quads in B+J mode, let's consider what can be done;
Typically running the 4 Quad fins as far apart as possible and larger fins at the front will deliver the maximum drive available from the board as supplied, which for B+J (especially on less wavey lake water even if choppy). Twins will simple provide less lift, less drive, less directional and a more skatey ride (great if you want it but not what most would consider a B+J ideal..)
As you're running the Q87 toward the upper end of the sail SIZE range, that imples it's being used in almost "marginal" conditions for such a setup anyway(or by a larger rider orand/or a particulary power hungry one etc), and that the (optional) use of some larger fins (than stock) may offer some further performance upgrad ( Maybe I'd redefine that as "rideability" upgrade??) To what size will in turn be influenced by your rider weight (unknown to us ??) but consider that for larger riders using something like 15/11 15/12 or even 16/11 for larger riders would be a possibility. Also depends to some extent on the make/model of the option fins (vs size) as yes they do differ vs mechanical size too. Note that that is not what we recommend for the board as intended (pure power wave riding) - but rather what you could consider optional (if available) to try and make the QUAD 87 into a more B+J tuning for lighter 87 wind range. On a lake...
Whatever you end up with, you'll at least have some good fun checking out all the options as well !
Cheers ~ Ian