I'd skip on tubular tires for LD events. If you get a flat and have to replace the whole tire, you'll have to glue the tires on, and that takes longer than swapping a clincher tube/tire. And there goes your time advantage....
Aero wheels will give you a slight advantage. The main question is: are the potential downsides worth those advantages?
The primary issue is one of comfort. For 100+ miles, you need wheels that will absorb a lot of shock and give you a comfortable ride; a wheel that feels fine at mile 70 may feel too harsh at 100 and beyond. The 303's are pretty low spoke count, so chances are it will be a harsher ride than 32 or 36 spoke wheels.
Overall my feeling is that unless you plan to do LD events competitively, or it's critical for you to come in the top 50 of a given event, for this specific purpose 303's are not worth the expenditure. Unless you're already close to your top form, you have more to gain from training than from swapping wheels.
Also, LD rides don't necessarily need to be TT's, particularly during the daylight hours. I'm not aware of any centuries or brevets that ban drafting, and have heard of plenty of people who hop onto a paceline during an event.