Rusty-c:
Let's put this in perspective: My tandem has 700cx25s. I'm "only" 195, but add in my son, 105, and the wheels on our bike are seeing over 50lbs more than you're are with just you and no one stresses with a tandem at "just "300 lbs. Put my little brother on, 160, and we still don't stress. Even with my neighbour, 275 lbs, we didn't stress on the tires We did stress a little on the drivetrain up one steep hill that put us in low gear...470lbs on 30-34 was putting some load into the rear chain/hub.
Yes, the tire flattens a little but I have never given it a second thought.
As DieselDan points out wheels are probably greater risk...eventually you will break spokes/crack the rim on the stock wheels. As jaxqtr points out by maintaining proper tension, you can delay this but the wheels are designed around 170 lb riders...the tandem has better rims, more spokes and a better hub.
Don