Its interesting, we I use 100LL I usually get 25 hours on a set of plugs.  I wonder what factors might make the difference between your time interval and mine - "Plugs Up"?.  I have not run across any chart that indicated anything other than a plug digit which indicated a "hotter" or a "colder version".  I think I may try  hotter plugs and see if they make any difference.

 

Ed 

Look at an old rotor housing to see if there are tiny radial cracks around the trailing hole.

This indicates an overheated spark plug.

 

That 25 hours seems acceptable to me. I would try Autolite AR2592. Very cold heat range, good for racing. Thicker ground electrode. Order through Autozone for about $7.00 a box of 4.