What he said, once the initial TM backup is done it’s difficult to notice any difference in connection speed.
Have you taken the Lacie apart? You can probably just replae the disk inside with a bigger one. If it’s old skool PATA you’ll struggle to get past 500Gb though, and it’ll probably be cheaper to buy a 2Tb USB drive.
Regarding your backup strategy, sounds pretty good to be honest. I probably wouldn’t even bother with the SuperDuper clone, but I don’t have my work on my home PC. Just remember the backup isn’t an archive (necessarily .. though TM will go back a fair way) and have other copies of your work history copied off permanently and kept offsite. A rotation of drives with clones on them might be enough. Or look at something like Carbonite and automate the offsite process of the important stuff.