Is it worth upgrading to 28mm tyres from 25 for a 250 mile ride?
If you're going to change anything before the big event, change it now and allow time for any problems to arise, then be sorted out before the event.

Generally, a 28mm tyre is said to give lower rolling resistance over a 25mm and has been tested, though using a 28mm at the same pressure as the 25mm could spoil some of the comfort advantages. The numbers tell us that you could run a 28mm tyre at 20 psi lower than a 25 with very little performance difference. 

More significant might be saving a few more watts with tyre choice to a different model in 28mm, but beware increased cost and (possible) lower puncture resistance.

So, if there's little difference in performance, or even an improvement and a 28mm proves to be more comfortable for you (higher volume, lower pressure) then you're looking at a win-win. Just watch out for frame and fork, brake, clearance for the larger tyres. Another reason to test now, not in a month's time.

Up to you if you want to spend the money, but I would try it for yourself rather than focus too much on what you can find written on the internet, some of it may be written by advertising experts for a start.