The short answer is no, not without modification to the header pipe on the bike because its 4" longer than the header pipe on the 250R.
I cut the extra 4" off with an angle grinder and cutting disc then fitted a Delkevic CF slip on that I had on my 250R previously. It didnt quite line up 100% but its pretty close.
Would you be willing to show some pics of the section you cut on the slipon? I really like the look of the Termignoni slipon for the 250 and I'm considering doing the same thing you did haha. I'm a sucker!
I just snagged a Yoshimura TRC slip-on for my 300F, meant for a 2011-2013 CBR250(and many other make/models), off craigslist for 100$. Had to cut 3-4 inches off the header and turn the mid-pipe a tad on the muffler to make it fit. I'll take some pictures this evening.
That thing is LOUD, it definitely quieted down a bit after the first day or so but I went ahead and ordered the low volume insert.