Yeah, air and decay is an appropriate description. I think it's due to lack of or less resonance due to the cup design and material.
Yes, but only in a shop. I find the G-cushion just creates bit more bass and/or less treble, and I don't find any need for more bass on my SE325is and RS1i, nor do I like less treble. If I want more bass I'll just use bass boost. Soundstage I find it increases slightly, but honestly I think it's just due to the driver getting farther from my ears and having more "echo" effect due to bigger and more sealed bowl. So there, more bass and soundstage, in a nutshell, at least to my ears. Again, I'm very particular in "sealing" the pads on my ears.
Why not get something like what I got? MS2i with G-cush pads... They deliver decent base, comfort, and all around larger soundstage, without the brightness of the SR325.