I've done this mod to the stock pipes on my old 650, here's the youtube vid.
I kept an eye on the spark plugs and all for hundreds of miles after, never had any issue, fuel efficiency stayed the same too, hence this mod has no measurable effect on the engine or fueling.
Sounds waaaaaaaaay better than the stock "sewing machine" sound, sounds like a real motorbike now.
Then I got an 1100, didn't want to start hacking it up because it was so shiny and new and perfect, but several months on (2 weeks ago) I said feck it, let's do this thing. Only drilled 4 10mm holes in the end of each pipe though, as I've just ordered a dynojet stage 1 rejet kit online today, so am going to completely (almost anyway) the stock pipes and want to be sure I'm not going to melt the pistons with a lean fuel mixture.
Made a video of the before & after, but haven't stuck it up on youtube yet.
I'm not impressed by the drilled pipes though - I've lost the quiet power the 1100 had... before I could take off real quick and nobody knew the bike was making an effort as it was pure quiet as stock, but now with the drilled pipes everyone hears the engine making an effort.
The fuel efficiency is still exactly the same as before, 142 miles to 12.5 litres (I always fill up as I'm just going into the reserve), so by that logic the fuel mixture hasn't changed.
I'd be very worried though if I just gutted the stock pipes altogether (stock pipes look better than any of the aftermarkets if you ask me) or replaced the pipes with aftermarket ones, and didn't rejet to suit....
From what I've read, there are 3 main types of exhaust.. a) very restrictive stock pipes (quiet), b) straight throughs (no restrictions) and c) performance pipes (small bit of restriction).
Apparently straight thrus (with correct jetting) do give noticeable better performance than stocks, but not as good as performance pipes. Apparently performance pipes have a small bit of restriction in them, in the right place & way of course.
But this is just a summary of what I've read, I know SFA about them myself.
Lowering the tone since 1969