Your seeing a good gain because your engine breaths better for a bunch of reasons, not just improved throttle area. I know I've piped up about this before but I think you'd make as much peak power with a single throttle. maybe you are seeing some better responce at part throttles because of your ITB's ? I don't know. I don't understand them much, I only understand stuff that makes me go faster on the strip so my opinion is a bit one sided
Glenn's car minus the gas is a good example of what is possible with a standard cam. It reflects in the trap speeds that people regularly see with that kind of tune (108mph there about). Glenn could have pushed that upto 240hp with a good camshaft and good headwork, still with a carburettor. It will have taken some carefull jetting. That would be a 112/113mph trap speed. Making more power than that with a carburettor is hard work. I don't know how much easier it gets with a big throttle or ITB's because I havn't tried though I think you've proven that there is a lot to be gained, just keep in mind that some of that efficiency will have come from your camshaft and cylinder head too.