Pickups don't have much to do with sustain...although some people say that alnico 2 pickups allow the strings to sustain a little better than alnico 5 pickups because the magnetic field is softer with alnico 2.
The Custom Custom is an alnico 2 pickup...so there ya go.
I can assure you that the Custom Custom sounds awesome in a nice Strat, because I have one in my favorite single pickup Strat.
I have three single pickup Strats: one with a Custom, one with a 59B and one with a Custom Custom.
The one that gets played the most has the Custom Custom.
Alot of players compare the Custom Custom in a Strat to Eddie Van Halen's early "Brown Sound".