Thanks for answers. Yes, 38 Spl +P+ is available as commercial ammo, for instance Cor Bon with pressure 26000 psi, but I personally have not seen any. Of course even +P+ cannot give higher velocity than 357 Mag. But, let me repeated my question:
Ruger manual allows 38 Spl +P+ for GP 100, but NOT for SP 101, even though it allows 357 Mag. I do not know the reason. I suppose there are at least two factors in play - thickness of chamber walls and a gradient of pressure after initiation. But it is only my guess. So it remains a mystery.
I fully agree that SP 101 is quite excellent revolver, accurate, reliable and moreover, very nice a piece of stainless steel (a bit heavy, I must admit).