My 2c. Agree with original poster. I've owned a CBR250R since 2011. Now have a small stable of bikes all the way up to 1100cc. I still enjoy a blast on the twisties on the CBR.
I've used two aftermarket muffler for the CBR ... a 2 sisters I got when I bought the bike new, being a clueless beginner. ******************** thing just about sent me deaf. I was quite happy when it got smashed to buggery when I crashed bike on RHS. Recently been using a slightly beat up Yoshimura carbon I temporarily swapped with my stock over to teach a new rider. We practice at 6am in a public car park with houses around. Would have the cops sent around with noise complaints if he used the Yoshi that came with the bike. That thing is again obnoxiously loud, have to wear ear plugs to go anywhere near it. The 2 sisters had the silencer baffle, but it did bugger all.
Interestingly I have ridden the CBR on the track maybe 30 times, and I have swapped between the 2 sisters and the way heavier stock several times over maybe 20 days, sometimes between sessions. I can categorically confirm that the bike hits a higher terminal speed down the main straight of SMSP with ... wait for it ... the heavy stock muffler ... over the 2 sisters. About 15 kph in it consistently. I can lap SMSP at a consistent pace, usually within a second if traffic doesn't get in the way.
But I do agree the stock muffler looks ********************. So I painted the shitty silver heat shield a high temp black, and it looks way better. That fat muffler just kinda disappears into the swing arm.
The 2 sisters was a carbon, it looked OK, the carbon Yoshi looks better, super sweet. It just fookin deafens you and I'd bet my bottom dollar the Yoshi also wouldn't improve the bikes performance without a dyno tune / power commander.
It really depends if you are a style over substance sort of person ... and even if you lean style - a CBR250R is unlikely to impress many ladies regardless of exhaust. For that you need an even louder Ducati.