YBR07

Hi to all, I had a search of the forum but couldn't find a specific answer - so thought i'd post and see if anyone had experience of adding a toth the the front sprocket of the YBR 125. This is my first post. I've an 07 YBR 125 and am considering getting 1 tooth extra front sprocket. This is to try to increase the top end (hoping for 5mph extra - where wind isn't against me). I'm also hoping that it will allow me to go a bit faster in the lower gears. for example, at 40mph i'm very comfortably in the top gear. With an extra tooth on the front I'd like to be able to use 4th instead. The same applies for most of the lower gears. So, here are my questions. Does anyone agree that this change would be beneficial? anyone think of any pitfalls? i expect to lose some acceleration, but it can't be that much as i commute through london so accelerating ahead of cars at lights is quite important! Also, a technical question - can I just swap out the sprocket, or do I need a new chain as well? I've had different answers from friends on this. I need to know what to ask for when (if) i take it to my local dealer to get the work done. Thanks for any advice that you can offer. Jay