Simultaneous Download Speeds On Virtual Private Server (VPS)

I just got a VPSlink account about a month ago and for all testing purposes, it has been great so far [my first VPS]. But I haven't transferred my main site to it yet mainly due to one specific question about download speeds. Quickly, here's what I've found about their network...

VPSLink is owned by Spry, so from all I can tell, they're using the same network speeds. Which are:
Unmetered: 1.5 megabits per sec in total traffic.
Metered: Spry's site says "throughput of up to 8Mbps"

Ok, so here's what I'm trying to find out:
I'm running a small software business with some downloads (all legal of course - they're mine ) and would like to know what the real-world download speeds that users would normally see when downloading my files. Here are the knowns:

a) Let's assume for this question that all of the user computer's are on very quick connections, so their speed wouldn't be an issue.

b) An example file would be say, 20 MB.

I'm most looking to see how speeds would be (in Kbps) if say, 20 people are downloading a 20 MB file at the same time (again, with their connections being extremely fast; fast enough for the sake of this example to not be a factor - so the max speed would only depend on my VPSlink server's connection).

I don't know of a good way to really test this since I only have access to 1 physical connection. I did try a test with a friend. Here's what happened:

I normally get about 500-600kbps sustained on my cable modem, as does my friend. When downloading a test file on my server I was able to get about that when downloading all alone. But, when my friend and I downloaded it at the same time, both of us got around 350kbps -- so the total speed about dropped in half. Again since my site is for software downloads, and when releasing an update to one of our products, sometimes I'm guessing 20+ users download simultaneously for a couple days.

This currently seems to work ok on Dreamhost, but for many other reasons, we're needing a VPS. But would this vastly decrease our file transfer download speeds to users? If users got something horrible like 2kbps, that would most certainly loose a lot of business for us since they'd just get frustrated and cancel for the most part.

Does anyone know of a rough estimate on how to figure this out, or even better, are there any VPSLink customers out there that have experience with this?