I thought of another significant factor.
As I mentioned already, if I use a 5S crank and keep my 3S rods, then the crank will have to be machined to suit the smaller big end of the 3S rods compared to 5S rods (I've read that it's a 4mm difference). This means that it may be possible to get an offset-grind on the crank to get even more of an increase in stroke, up to a maximum 95mm stroke. If I used a 5S block, 87.5mm pistons and an offset-ground 5S crank with 95mm stroke, then I would have 2285cc displacement!
However, the pistons would have to be very much a custom item, as there probably aren't any off-the-shelf pistons to suit this setup. The wrist pin would need to be 2mm higher compared to a 5S piston, and 4.5mm higher compared to a 3S piston.