hi,, here is my two cents,, by using the proxy, your going thru the server on which it is held and not directly to the internet backbone on your business account. so the proxy is NOT EXCLUSIVE to you, so this will slow speeds down because it is a shared server. you should get better speeds late at night then during the day..
also i see little point in using the proxy as you have the employees access to certain items already blocked.
how many computers share the internet access route? if 4 then you need to divide 16 by 4 which is 4 and deduct 20% of that for what microsoft keeps open for its updates and file server access.. so 3megs is about right,,