[DeTomaso] NPC - Brake Drum Wear Limits, does Porterfield R4S material eat drums, etc.

[Guido deTomaso]

 Have done a little driving with the Porterfield rear pads installed, to match the fronts.  While the fronts made a huge difference, not really feeling a big difference after changing the rears.  Ought to get out in the rain though, make sure the fronts are locking up first.
Is there any downside to this R4S material, other than initial cost?  Short life?  Eats rotors or drums?  Otherwise seems too good to be true.
I also installed the four shoes I had relined, drum brake non-Pantera application, new drums too since the shoes cost so much, not really seeing yet the huge improvement like I saw on the Pantera.
The old drums have a max diameter shown as 9.114 inches, mine are at 9.095 .  Thought I'd get them turned, swap them back on someday, but my guy says 9.114 is the wear limit, not the machining limit, so he can't take anything off.  Internet research seems to support both positions, both you can and cannot machine/turn to the max diameter cast into the drum.  Any thoughts/experience?
Also what's the minimum a brake lathe can take off typically?  If .010 on the diameter, that would put me at 9.105 with .009 leftover.
Same ideas would apply to rotors ... Pantera rotors too ... is the minimum thickness a "machine to" number, or a "wear to" number?
Thanks,
GD
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    Have done a little driving with the Porterfield rear pads installed,
   to match the fronts.  While the fronts made a huge difference, not
   really feeling a big difference after changing the rears.  Ought to get
   out in the rain though, make sure the fronts are locking up first.
   Is there any downside to this R4S material, other than initial cost?
   Short life?  Eats rotors or drums?  Otherwise seems too good to be
   true.
   I also installed the four shoes I had relined, drum brake non-Pantera
   application, new drums too since the shoes cost so much, not really
   seeing yet the huge improvement like I saw on the Pantera.
   The old drums have a max diameter shown as 9.114 inches, mine are at
   9.095 .  Thought I'd get them turned, swap them back on someday, but my
   guy says 9.114 is the wear limit, not the machining limit, so he can't
   take anything off.  Internet research seems to support both positions,
   both you can and cannot machine/turn to the max diameter cast into the
   drum.  Any thoughts/experience?
   Also what's the minimum a brake lathe can take off typically?  If .010
   on the diameter, that would put me at 9.105 with .009 leftover.
   Same ideas would apply to rotors ... Pantera rotors too ... is the
   minimum thickness a "machine to" number, or a "wear to" number?
   Thanks,
   GD