From Michelin, although a pretty long time ago and I have no idea if their policies have changed since:
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Subj: Reply From Consumer Relations
Date: 11/22/2002 11:47:55 AM Eastern Standard Time
From:
Webtire@us.michelin.com
To: XXXXXX
For tires with a speed rating of 130 MPH and higher (H,V,Z) only one (1) ¼"
(6MM) diameter puncture may be repaired and allow the tire to retain its
original speed rating.
It is essential that a trained tire dealer or repair person deflate and remove
any tire from the wheel when it has been damaged or loosing air. A thorough
inspection for any internal damage must be made. Never perform an outside-in
tire repair (on the wheel). For proper repair procedures, refer to RMA's
Puncture Repair Procedures for Automobile and Light Trucks.
If you have additional questions, please respond to this email
or you may call us at 1-800-847-3435 (toll-free) between
8:30AM and 6:00PM Eastern Time Monday through Friday to allow
one of our Consumer Relations Representatives to assist you.
Michelin North America
Consumer Relations Department
(Please Do Not Delete the line below.)
Email ID: 862819
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FWIW, I have done several track days with a properly plug/patched and rebalanced rear Michelin PS2 on a 993 C4S with no issues. YMMV.