ffkiwi pretty much sums it up well. I've used both the standard plug and turbo plug conversions he mentioned sold by cox hyper speed on the auction site. They run consistently but with varying amounts of power/rpm depending on the plug style used just as he says. Sometimes a slightly detuned .09 is just what you're looking for on some models. I have a Medallion .09 that has run many hours over the years on a standard plug conversion on an old Randolph-designed Pong Two. For optimum performance, the stock Cox heads have been the ticket for me. A good hot turbo plug conversion is a little bit below that power in my experience. Haven't tried the Nelson plugs.
I tend to run the conversion heads the most since I only have a handful of stock factory units and they are getting harder to come by(as are the great rubber ducky 7x3.5 props). Seems .09 pilots are getting fewer all the time. Parts supply is sometimes a challenge for these. I love their loud, raspy sound!
A video of my Medallion .09 flying at the SMALL fly-in a couple of years ago. Had a stock head on it in this clip, though.
http://www.youtube.com/watch?v=B_MV4PEy6jI