I've been running 25s on my mt R1850s and it's getting to be time to buy another set of tires. I ride on a lot of chip sealed or just rough tarmac. It is worth the $$ from a comfort standpoint to get a set of wheels built to run 28s? I'm 6'4" and weigh 200+ so I have to keep my tires hard or I pinch flat (or in the case of my tubless MTB bend rims) .
I'm pretty skeptical that they would help much but thought I'd ask for experiences.
28mm tires will always be more comfortable than 25s (unless you go from a super-supple race tire to an armored tread tire). You don't have to have new wheelset built to use 28mm tires. The question is whether they will fit your frame and brake calipers.