In my experience with 16" wheels (Brompton and a Merc), I would avoid 20+ mph speeds. There are just too many surprises that can destabilize the small wheels ... mainly big pot holes.

The Curve has a 3 to 4 inch shorter wheelbase. I don't imagine that helps matters.

How fast can you spin? If you can comfortably spin at 90 RPMs then if you let the bottom end go to something like 30 gear inches--say with a 16 tooth cog in the back ... I believe that the SL has a 46 tooth chainring--then in the top gear you could still hit ~18.5 mph.