ASYNC Bandwidth (cable modem) Theoretical Speed Limit

is it possible to actually download at 100MB if you cannot acknowledge faster than 5MB?

Yes.

It is possible that the upstream speed could be limiting, but that would be using some primitive scenarios where you are acknowledging very small blocks of data.

Let's use a 1,460 byte window size (which would be equal to a 1,518-byte ethernet frame).

The sender is cranking out no more than 8,234 frames per second (100m bits per second/8 = 12.5m bytes per second/1,518 = 8,234 frames per second).

Now if you consider that an acknowledgement is a minimum segment size which would work out to a 64-byte frame, that would be 512 bits per acknowledgement. Which means that with a 3mbp/s circuit, you could send 5,859 acknowledgements per second.  So in that situation, the sender would slowed down to about 71mbp/s... if we were talking about a 1,460byte window size. Which is possible, but more than likely your hosts will be using sliding windows to allow a window size much larger than that.

So now (for example), let's increase the window size to 4,000 bytes.

That means you only need to send almost a third as many acknowledgements. Which results is receiving three times as much data (210mbps with a 4k window).  If the window size is larger, you receive more data per acknowledgement.