In 1995, STATS LLC began tracking the times cornerbacks were targeted and the yards and catches they allowed. According to those stats, Deion was far and away the best shutdown cornerback in the 1990s. Nobody else was even close. And the only guy who has put up similar numbers since then has been Nnamdi Asomugha.
Here's what Deion allowed from 1995 to 1998, according to STATS --
1995 -- 16-for-47, 158 yards, 0 TD (9 games)
1996 -- 22-for-46, 225 yards, 0 TD (16 games)
1997 -- 12-for-39, 159 yards, 0 TD (13 games)
1998 -- 20-for-42, 198 yards, 0 TD (11 games)
Compare those to these stats for Champ Bailey, whom some people also consider to be a "shutdown" cornerback --
1999 -- 54-for-113, 830 yards, 4 TD (16 games)
2000 -- 42-for-81, 488 yards, 0 TD (16 games)
2001 -- 40-for-91, 613 yards, 5 TD (16 games)
2002 -- 52-for-98, 567 yards, 2 TD (16 games)
2003 -- 50-for-88, 684 yards, 3 TD (16 games)
2004 -- 49-for-83, 768 yards, 6 TD (16 games)
2005 -- 49-for-97, 680 yards, 5 TD (14 games)
2006 -- 50-for-95, 610 yards, 2 TD (16 games)
You also can compare Deion's numbers to those of the other All-Pro cornerbacks from 1995 to 1998 --
1995
Deion Sanders -- 16-for-47, 158 yards, 0 TD (9 games)
Eric Davis -- 57-for-105, 705 yards, 6 TDs
Aeneas Williams -- 45-for-93, 774 yards, 5 TDs
1996
Deion Sanders -- 22-for-46, 225 yards, 0 TD (16 games)
Ashley Ambrose -- 53-for-105, 715 yards, 3 TD
1997
Deion Sanders -- 12-for-39, 159 yards, 0 TD (13 games)
Aeneas Williams -- 41-for-92, 534 yards, 2 TDs
1998
Deion Sanders -- 20-for-42, 198 yards, 0 TD (11 games)
Ty Law -- 55-for-119, 598 yards, 2 TDs
No, Deion was not overrated. Not one single bit.