I was seriously considering doing the same thing. I wound up going another way but I still think it's a good approach
The difference between the 7.62 and 12.7 cans on-bike (I was just sort of bracing 'em near the bike) is actually pretty small. On my bike both sizes are totally behind your legs as you ride so there is no real drag difference. The difference in usable size is huge. I had a hard time fitting even a pair of gloves and some bike basics (oil and whatnot) in a 7.62 can. The 30 cans tech used wouldn't fit on my bike with the stock exhaust system. He's lucky that BMW of his has those nice straight exhaust pipes.
You want the boxes as far forward as possible. That's probably more important than weight really. As an anecdotal example of this: My boxes are very far forward ... in fact they take over the passenger pegs and my heels can hit them if I put my tiptoes on the front pegs. I put them as far forward as I could without impacting ridability. Just after I mounted them I saw someone on another forum who had mounted givi hard luggage on the same model bike. A very nice install too... he welded up a bracket for the givi mounts and had two suitcases and a top case just like you'd see on a v-strom or the like. All of it was positioned "normally" i.e. back enough that the passenger seat was still usable. The difference? His looks just like a small touring bike...but after testing he reported that with approx 30lbs (total) in the suitcases the bike was almost unridable. Very light in the nose, unstable, bottoming the rear suspension, etc. I've had 50lbs of stuff in my bags (25lb/ea side) and the handling difference was hardly noticable... it was like having a very light passenger. Same bike, same goal, maybe 16" difference in position.... huge difference in the end result.
If you are going to do this, do it with style... find a place where the cans fit aesthetically... and don't hesitate to do your own thing. Many welding shops (including independent muffler shops) will do fab work like that as much for the word-of-mounth advertizing/joy of doing it as for the $$$. Find something that fits the bike and what you want to do with it. For example... consider extending the bottoms of the cans (the right welding shop can cut the bottom off the can and weld on new metal to make the can deeper, angle the bottom, or create a side extension to increase the volume without changing the lid size, though it'll be $$$) to fit the lines of the bikes... or mounting just one can on one side... or whatever looks good to you. Just make it obvious you meant what you did. Some people may not get it... but many people don't get bikes to begin with so you should be used to that by now.
And have fun! It's now "custom".
Ride it like you think owning it matters.
