Well i have tested the speed limits in a LAN enviroment and they seem to work fine. There are 2 factors that might be responsible for your observations:

1. As you probably are on some kind of ADSL line your upload and download speed are "linked" together.
quote from a posting on some msg board

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First, ADSL is 2 channel as much as SDSL is, except the channels are different sizes: upload is usually quite a low speed.
Second, its got nothing to do with the remote server having trouble doing two things at once.
The main reason upload slows download and vice versa is that downloading at full speed takes some of the upload channel for ack packets. Not a lot, but some. About one small ack packet for every 2 large data packets.. With really unbalanced ADSL speeds, like 1500/128, this small up traffic of acks is actually quite a chunk of your 128..

The bigger problem, that people have observed and emailed me about, is to do with TCP throttling down when there are delays in ack packets.. as would happen if you were hammering your upload channel.. the end result is that you cannot optimally use full speed in both directions, even on SDSL lines, for TCP protocol (used for FTP/http etc) although you can for UDP traffic (streaming video)..
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2. the people downloading from you might only be on modem or ISDN