I’ve had my bike for a year now and done 4,000 miles on the beast. I’ve got to know the way it feels and how it sounds and performs. It’s just had an annual service and had a few upgrades done at the same time. New brake fluid has give me sharper brakes. I’ve swapped the Pirelli Phantoms for Avon Spirit ST’s. These improve the handling enormously. I’ve also de-catted the thing and replaced it with a TEC X-pipe. It makes a noticeable improvement. The biggest change is in tractability at low speed. The bike now pulls strongly a lot lower down and is considerably less hesitant. Full bore it still feels fine but any improvements are not really noticeable to old bimblers like me. I’d also argue this is not quite why you have a Bonneville but it certainly isn’t slower. The X-Pipe now means she barks properly as well. I won’t wake the dead but it does sound considerably better - more musical if you like.
Have you noticed any changes in MPG? I'd love to do this on my T120 but not at the expense of gas mileage. I already took a hit when I traded from T100-120.
Over the last 200 miles I have measured that mileage is factionally improved (63.4 up to 63.5 mpg). But this could be more to do with the roads and me driving slightly slower as I get used to my new tyres. I think we’ll find overall that there is no significant change or a very (very, very) slight improvement.