Restricted surface area is not the only issue. As I understand it, the air flow around the top rim of the trumpet is important so by fitting these tea strainers you will alter the air flow and lose some performance.
But having said that, I've been using these tea strainers now for a few years with some track work too and the difference in performance is not dramatic. I couldn't "feel" the difference but it probably could be measured on a dyno or with a stop watch.
The question about grit and the reduced life of the engine is a good one too and was something that did play on my mind. I also get asked the question regularly so I'm constantly being reminded. But I figure if it lasts 50,000km between rebuilds, that would equate to six years or more for the amount of driving it gets. And lets face it, these engines in road trim don't cost all that much to freshen up. Incidentally, I have done about 25,000Km so far after three years and the engine is just starting to feel like its loosening up, so I think it has a long long way to go before it needs a rebuild (I hope these aren't the famous last words
).