Any question answered...

Would it be a worthwhile upgrade to go from the 2005 Bontrager Select wheels I currently have to a pair of Shimano R501s.

I weigh about 14 stone and these would see use commuting so need to be fairly resilient...

R501s are nice, but I'd be surprised if they were actually an 'upgrade' from anything which is already circular, and while surprisingly robust they wouldn't be my first choice for a 14st commutenger. Get some handbuilt wheels, e.g. Rigida Grizzly + ACI spokes + whatever more or less equal sealed bearing hubs are on offer today.