A 2.5:1 overdrive Schlumpf drive will introduce drag that wouldn't be there with a large chainring and small tooth cassette and will actually slow you down due to the inefficiency attempting high speeds.. 1:1 will be the low range and 2.5:1 the high range ..

There are few ways to get to 100gi on 16inch wheels.. without going to massive chainrings.. one that I've used is a Capreo rear hub with a 9t small cog.. the hubs are relatively inexpensive and the smaller cogs replaceable ..

Another bike option is a used Birdy.. wheels are 355mm diameter, and compared to a 349mm Brompton wheel, are only 6mm larger diameter, putting it on the larger end of what is considered 16" wheels.. with the well thought out Birdy suspension, you would be able to maintain higher speeds more easily as road shock wouldn't be wearing you out as quickly.. Birdys also use pretty much industry standard drivetrain parts, so an upgear path is pretty doable..

If I was looking for a sub 406mm bike to go fast on, it would be a Birdy