For the most safety and performance, I suggest getting up to high-speeds gradually. Identify the fastest sections of the courses you ride regularly and each time you hit that hill, do just a little more speed, maybe 1-mph faster each time. You'll get a feel for how the bike handles, how it reacts to road bumps, how stable it is or isn't in cross-winds and you'll get the hang of making the necessary corrections.

The changes in behavior are fairly linear with speed, so the change from 40 to 41 to 42 to 43mph are predicatable and managable. However, if you've only gone 40mph down a certain hill before, then bomb it at 50mph the next time, the sensations and required corrections will be very dramatic and scary.

One of the most difficult things to develop is a firm grip with relaxed arms. Your fingers are tight on the bars, yet your wrist and arms are loose to absorb road-bumps and to provide quick responsive action to make corrections. A small bump that requires a 0.25s response time and correction at 40mph to stay on the desired path, will give you only 0.17s at 60mph.