Well this escalated quickly!

I will try to answer everything in order.

I dont think the 531 head will be providing much of a restriction to a 16t at 15psi. The 531 is favourable to the other castings due to the shape of its combustion chamber and the location of the spark plug. If you were going to the trouble of cutting the seats then it may be beneficial to have a look into installing larger valves while you are at it. @AshDVS can sell you the larger valve kits which will definitely increase exhaust flow and help the 16t get onto boost quicker. That being said, the 16t is a small turbo and will boost easily without much encouragement. For a daily im not convinced it would be a worthwhile spend. Shaving the head too much can sometimes result in the cam timing being put off or the belt being loose because the head now sits too low. The solution ive seen to this is to run a thicker gasket. I have no personal experience with this so take it for a grain of salt. In all honesty, I wouldnt be keeping that 531. It appears to have some serious corrosion going on and a healthy 530 would perform just as well when being controlled by LH2.4Yes there is a difference in the NA and Turbo pistons. The NA pistons have a shallower dish in them and less material where the top ring seats. This isnt usually a problem until you experience detonation or excess heat and the piston cant get the heat away from the ring. If you were to swap in turbo pistons then i would consider not running a 531. Your compression ratio would be lower than that of a stock B230FT which from memory is 8.7:1. If you swapped in the turbo pistons then it would be a good idea to install a 530 to retain the compression ratio. If the NA pistons are fine then I would keep those. If you can manage the heat and detonation then they will live. I dont think you will need to have it balanced as I think each part is individually balanced and not as a set. That being said, it could help find any causes for vibrations in the assembly that could get into the cabin. The B230 isnt a refined engine by any stretch of the imagination.

Hope this helps!