I think the #4 teat is the fastest in the Avent bottles. There is nothing wrong with putting an extra hole in the teat, to make it faster for your lo, it will just make the teat deteriorate a lot quicker than normal, and would need to be replaced sooner. Avent do a variflow teat thought, so that might be an option.
I agree with Mashi, your lo could be starting to teeth, and if they are moving up in his gums that can be very uncomfortable for him. I would try some teething gel a few minutes before a feed and see if that makes any difference.
Keep us posted.