Well I don't have any pictures unfortunately, I could get some once I get home in about a week.
I can explain the procedure however. The absolute easiest, and in general the best way to do this would be to disassemble the rifle (You don't want any steel shavings inside of it). Remove the barrel, clamp the rest of the rifle vertically in a vice with some rags. Then drill from the front (preferably) with a drill press. This could be done in less than an hour easily.
I however had to resort to a much worse way of doing things. Basically the bolt that attaches the barrel to the rifle is of really REALLY bad quality, so I stripped the damn thing by barely looking at it, meaning I couldn't remove the barrel anymore. I solved this by welding a 3.5mm drill bit to a thin steel rod and using it as an extender to drill from the other side. I think the results are the same but it was a real hassle.
While I strongly recommend going for option one, I also have some general advice when doing this.
1: The distance between the end of the piston and the actual transfer port is really long, so be prepared to drill through something like 20mm of (admittedly very soft) metal. There really isn't anything on the other side you can damage as long as you disassembled the gun.
2: It's much better to drill a hole that is too small than too big. While it isn't really that hard to drill out the transfer port and thread it and put in some brass "adapters" to change the size, it's much easier to just not make it too big. 3.5mm works well for me, but I can't promise it is optimal, just that it is much better than stock in pretty much every way. A bigger or smaller one might be even better, I don't know but I'm happy with what I got so I'm not messing with it.
As for real world numbers, all I can say is that is is much more powerful and accurate. I assume since the gun is designed to work best at its high power. While I don't have an really accurate way of measuring the velocities, I have observed that they have gone up substantially. This is most noticeable when shooting at something like 60-100m. Before the rebuild the gun would be completely useless but now the drop is quite modest. Using just the slow motion camera in my phone and counting the frames for the projectile to travel 100m, it has gone from roughly 170-185m/s to about 280-290m/s. Accuracy for me is about an inch at 30 yards, so really quite good, but that's also after building a custom trigger and a tune.
Hope this helped!
