Many thanks for your quick reply. I've heard about ATI's relegation of "old" cards, which I gather also causes issues with power management (and therefore heat), particularly relevant to laptop users.
I tried removing console-tty1 and the splash, video and vga strings mentioned in the article to which you pointed me. If I leave console=tty1 in, I can't boot at all, so that has to come out. When I take out the other strings as well, the Sabayon splash disappears but I still get all the messages throughout the boot process up to the login screen. So the only apparent affect seems to be to change the background to nothing (whereas the sabayon splash gets overwritten if I leave it in), and I still get the verbose messages.
I'd welcome any more suggestions for this annoying "feature". It doesn't affect operation, but I'd quite like to sort it out before I change several of my friends to Sabayon. The plus features of Sabayon make it particularly attractive to me, but this visual thing will probably sap the confidence and trust of some of my more nervous friends....