how much of a difference does bike weight make when trying to maximize the bike's top speed?
The weight of a bike has virtually no effect top speed. It takes very nearly the same amount of effort to maintain a speed of 30 mph on a 20 lb bike and a 30 lb bike on flat ground. The biggest factors that come into play at high speeds are aerodynamics and rolling resistance ( slick, efficient tires ).

http://velonews.competitor.com/2012/05/bikes-and-tech/technical-faq/tech-faq-does-bike-weight-matter_220429

That said, is top speed what you're really worried about? Probably not. Even if you like to bike fast, I truly, truly doubt you should be concerned with top speed. You'll probably enjoy a bike that accelerates quickly and feels nimble. Unless you live in a flat, remote area, in which case you probably wouldn't want a single speed, you're probably going to be accelerating and maneuvering more than you are achieving top speed. And again, these factors don't necessarily correlate with the weight of the bike.