I decided to slip into the slow lane and installed sp7 on my kids' machine. It's a 400 MHz blue-and-white G3 with 640MB of memory running OS X 10.4.6. I don't have a "minimal" memory machine. Yes, it felt very slow, but it was doing okay (even with loading that Million Dollar Home Page). I tried to use up all the memory using other apps as well (Safari, iPhoto, Enigmo). OW hung out with about 100MB of real memory. Yes, it would be nice if that were a smaller footprint, but things were happening.
I wandered around trying to load pages and eventually got to a page where Activity Monitor showed OW going in and out of responding. But now I see that the "page" I was trying to load was a PDF that did eventually display on the page:
http://www.nso.edu/general/observe/sched/dst_may06.pdf
There may be something funky in that file because it's taking a while to display it in Preview or the Finder's inspector panel on my dual 1.2 GHz G4. On my G4, I download PDFs to the desktop, so I'm not sure if this is a bug only on slower machines or with the PDF in-browser rendering.
I never managed to get OW on my G3 completely back from trying to display that document. I ended up Force Quitting after taking various samples and pictures.
But I'm one data point for using OW5.5sp7 on a lower end much older Mac. I'm not saying there isn't a memory leak, but it is usable and the Million Dollar Home Page didn't immediately crash it or soak up enormous amounts of memory. It stayed up longer than Firefox seems to on that machine. So maybe something else is happening in your system.