Well Godfrey
I use aperture on an iMac with a 1tb hard drive, on which I have all the pictures I've kept since around 1990. Currently I'm using 10,12,18 and 24mp RAW files, and I simply don't notice the difference in processing speed.
On the other hand, I certainly do notice the difference in IQ between the larger and smaller files, even when printed small. Add to that the cropping ability and the possibility of printing that accidental 'shot of a lifetime' really big, and it seems to me that the overheads of large files are meaningless, but the benefits are very meaningful.
You speak like a man who has settled for 12mp and is feeling faintly uncomfortable!
All the best