Well, maybe 80% of 54 Mbps. The other 20% is overhead. But your right, the smallest pipe wins.
Desktops and laptops are not the same by any means. The disk drive speed is usually 7400 rather than 5400 RPM. Processor speed may be different and a host of other things.
Also Windows, is crippled (I know XP is). It can't download faster than about 15 Mbps without some tweaks. With tweaks I got 36 Mbps on an essentially 100 Mbps local infrastructure running on essentially infinite bandwidth.