I have both billets (intermediate for the 1-2 shift as well as 3-2/4-2 downshift and the overdrive for the 3-4 shift), the difference in holding capacity is around 40-50% over a stock servo and about 35% over a 'Vette servo. The OD billet holds around 50% better than OE.
Now, firmness is pretty stout with the billet intermediate one. BUT if you keep your 1-2 accumulator washer free (with a TransGo shift kit) and the 1-2 feed hole small you'll keep the shifts still authorative but not harsh.
I'm going back to tone down my 1-2 as I drilled my 1-2 hole to around .082" and I'm going to make it about a .063-.070" which will make the shift less stout but still have all the holding capacity. I'm also installing a hardened TransGo seperator plate along with pinless accumulators instead of my aluminum pinned ones.
Also, you'll want to upgrade to a piece from Sonnax called a Servo Release Check Valve (with either 'Vette or Billet), this prevents tieup on a 3-2/4-2 downshift...tie up could eqate to burned 3-4 clutches which is no bueno.