well with that setup i don't think you will be needing the bias valve
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the surface area of the pistons on the front will be quite large compared with the standard giving you a larger front bias ,I run 996 4 piston calipers all round and really like the feel of them , like you said they are not grabby at all plus mine were from a rear engines porsche i would think was more suitably set up for a rear engined car.Im sure mr porsche did all the calculations for me .
To use the bias valve you will have to slightly over bias the back with larger rear pistons on the calipers or improve the grip on the front end to such an extent the rear looses grip first , personally i will be after a balance bar setup when i get round to the next stage in my cars evolution as when sorting the bias it is cheaper altering the bar with a spanner than swapping the calipers etc .
As for the m.c acording to my calculations i does not matter which way round it is just the 23mm piston will supply more fluide but will not alter bias, i use a 19/19 with my setup