There was an article in September 2005 Aviation Consumer. It covered the topic of porting & polishing. This is a process of cleaning up obstructions & roughness in the intake and exhaust ports. They also discussed the benefits of grinding the valve seats. The bottom line of the story was that you could gain up to 4 HP per cylinder, or in other words, 24 HP in a six cylinder engine.
The shop discussed in the article was Performance Engines, based at Brackett Field in La Verne CA (southern most end of California). Their website is:
http://www.performanceengines.com/
You should read the Aviation Consumer article. They address the very gray area as to the legality of this mod. It’s available online to subscribers at:
http://www.aviation-consumer.com