Seabreeze, unfortunately AI Gigapixel doesn't support TIFF with layers as I've mentioned before.
It can open TIFF with a single layer with transparency in some areas but the result is a flat TIFF with no layers.
The only way to have transparency is to use PNG though it's better to save the resized file in TIFF because saving in PNG takes an enormous amount of time, unacceptable in a profesisonal environment.
As for RGB I'm perfectly aware that the CMYK conversion is a final step, we're a prepress company.
Anyhow, we receive a lot of files already in CMYK from the clients and that's the typical kind of file that must be enlarged ASAP.
So it takes a lot of time doing all these workarounds (merging all the layers into a single one, converting in RGB, enlarging, converting back in CMYK, changing the dimensions, etc.).