No problem matey!
First off, the EVR airboxes are a really nice bit of kit and not actually that expensive for what you get - and they're certainly the "easy" option.
From my understanding, the only airboxes that add any real structural strength are the genuine Corse ones. The main difference is the section under the fuel pump is fully closed, just with a couple of holes for fuel lines etc. By comparison, the "stock" airboxes are really just a perimiter frame so although they may add a certain degree of frame bracing, they won't offer anywhere near the same amount as the Corse version. To be honest, I don't really think that you would notice a difference unless you're capable of running with the rest of the BSB field. Another point to bear in mind is that the 748R airbox is the same part as the 748RS airbox. If increasing chassis stiffness was really such a major issue, wouldn't Ducati Corse have used a "stiffer" airbox in the 748RS? Obviously the 748RS had to comply with FIM rules for Supersport racing but I think that they could have stiffened the airbox without breaking the rules (I'd have to check the FIM rulebook to give you the definitive answer on that though).
Modding the lower part of a 748R airbox will have zero overall structural effect. It just bolts onto the engine and then the top half (which bolts on the same as any other airbox) literally just sits on top of it with a rubber seal between the 2 halves. You'd obviously have to make sure that after you modded the lower part that it was strong enough to retain it's shape and keep the seal between the 2 halves doing what it should.
Another point to consider is that if you fit any larger airbox you're going to need to spend some time on a dyno and use a Power Commander or similar to get it running right. Overall, it's going to be a pretty costly mod and I'm not convinced that you would notice any difference. For my money, I'd be setting squish, dialling the cams, setting valve clearances properly and use a PCIIIUSB to independantly map each cylinder - that you would notice!
Cheers, Neil.