A lot of factors control your internet performance.
In reality, the modem is a minor player in this.
I believe that the SM56 is a controller-less modem. As long as you have at least a 500MHz CPU, and the CPU is not over-tasked with other duties,you should not expect a significant performance boost from a different modem.
The biggest factors are:
Phone lines between you and the phone switch, and between the phone switch and your ISP
The switch itself
How your ISP is setup to handle service
The connection from the ISP to the Internet backbone.
If you are getting a 48K to 52K connection to your ISP, any speed issues are between the ISP and the internet backbone.
Let's say that your ISP has a 1M connection to the internet.
That supports 20 users at 50K each.
To make a profit, the average ISP will have several times that number of users per 1M of bandwidth.
That means at peak usage times, you WILL see a reduction in data transfer.
The only way around this, is for you to pay extra for dedicated bandwidth.
Chas