Bilstein claims that their shocks retain something like 98% of their effectiveness at 100k mi., so barring any leaks (which would be visible as wetness on the outside of the shocks) they should be fine for a long time. If you've noticed a deterioration in performance, however, then you've answered your own question; sounds like a shock or two is getting weak.
I like OE stuff, but I'm sure that products from the OE suppliers (Boge, Bilstein, not sure who else)would be highly adequate.
BTW, how many mi. on your car?
"If everything seems under control, you're simply not going fast enough" --Mario Andretti
1995 E500 street car
1986 Porsche 944 Turbo S track car