I can't speak for the S250 but for the 300i the rollers are wrong. the bike by default is set up for MPG and by keeping the RPM's down at top speed it never gets to the pipe. You only see 135-140 KPH stock but with Dr Pulley 20x12 14Gr you should see 155-160 KPH once broken in and the motor free's up. Of course once you get the power and speed up to the Burgman 400 range you get the MPG in that range as well 60-65 MPG.
I agree though, as an upgrade it may not be worth it. So long as the S250 fits your needs that is. The handling of the DT 300i is fine. it seems to take bumps etc as well as my motorcycle and feels fine at interstate speeds. I have put it over pretty good on some twisties and nothing scraped so it seems fine there as well.