Only you can answer that, but for what it's worth I too have a 05 300HPDI with over 250 hours of flawless operation. I also have an extended waranty from Yamaha which came with the motor when I repowered. Extended waranties & service contracts are like betting in Vegas, odds favor the house (they are in the buisness to make money), but if things go bad, its really nice having one.
I have an 04 300 and mine went up after 230 hours and it cost me nothing because of warranty from factory and I have the extended but if I would have had to pay it would have been @ 5 grand. the injecters for these pigs are 700.00 each when you have to put 6 new ones in the warranty doesn't seem that bad.
When the 300 HPDI first came out there were a ton of problems. Lots of unhappy folks hammering away at their keyboards in frustration with them. Most of the ones with problems were in saltwater. Dealers hated them. Many refused to sell them. Not sure they ever got them really fixed...as I haven't seen one rigged on a new boat in several years. If I were you, I'd go check the threads over on TheHullTruth and BoatFix before deciding to NOT extend the warranty.