Join Date: Apr 2012
Location: Minneapolis, MN
Bikes: 1991 Eddie Merckx Corsa, 1991 Cannondale 3.0 Road Race, 2008 Bianchi Pista
No reason to buy 105s for FGSS in my opinion. Tektro makes functional and cheap brakes and levers, if I was buying new brakes today I would go with their levers and either their brakes or sora/tiagra. Buying decent pads makes more difference than specific models of brakes in my experience, pretty much all modern dual-pivot brakes will be more effective than your current centerpull.
EDIT: I reread and I guess you mean you have a single-pivot, which are typically even worse to use than centerpull. Any upgrade to dual-pivot will be very noticable, even on the lower end of prices.