What, if any, mods could one do to a Merc Racing outboard?  Any comments would be appreciated.

What, if any, mods could one not to a Merc Racing outboard?

To many to list here

Jerry's marine in Sulphur, Louisiana has the Most Successful Mercury Outboard Racing Team in the World, with the credentials to prove it.

http://www.jerrys-marine.com/boat-racing.shtml

Scream and Fly is your best source of info for hot rodding a mercury.  There are a number of racers and engine builders on their tech forums that can give you as much and more info than you could ever want.  Eagle One Racing is one of the top dogs in building you a hot, reliable motor.  He is also one of the very few I would want inside my new merc.   The main problem you are going to run into is getting someone to do it for you.  It is strickly a no no to mod the newer engines so most will not touch them for someone they don't know.  

There is not much in the way of quick, bolt on mods that can be done that would make a noticable difference.   Big gains in performance are gained by one of the pro builders that know what to do with a die grinder inside the motor.    One thing to consider also, like any engine, to gain top end power, you have to take away some bottom end power, something that you might want to consider long and hard considering this motor is on the back of one big, heavy Ranger.  I'm not real sure you will find anyone that knows their stuff that would consider moding the motor on that heavy of a boat.

Redline, is that new 300hp not fast enough for ya??