I have referred to this article quite often when the desire for a lighter bicycle gnawed at me: Ultracycling: Title

I will say that at Brevet distances extra weight does either slow you down or wear you out faster. However, I prefer being slower but well prepared on a brevet. On a supported century or double-century for time, then I'd shed as much weight and get as aero as possible while realizing that a bike less than 12% of my body weight is unnecessary. Thus my "fast" bike is a 23 lb. stiff alloy frame and my "brevet" bike is a 26 lb. compliant steel steed.

Therefore to answer OP's question, if your current bike is <12% of your body weight then you don't need a lighter bike to go faster on a century. HIIT is the answer unfortunately.