The 800 is a great computer and will do most anything you want. The only thing it can't do that the current devices can do is allow you to set up "live" tracking on Garmin Connect by pairing the 510/810 with a cellphone via Bluetooth. Personally, I don't want that, but it's just me.
Personally, I think Strava is a better site for seeing how you improve over particular courses and segments. MMR is fine, but it's more for discovering new routes and sharing routes with others. In addition to having all the functionality you want in one device, you'll also find the battery life of a Garmin device is better than doing the equivalent functions using a phone.