Not being able to feel the shifts means that transmission is in good shape. Mercedes automatic transmissions are extraordinarily good.
When you floor it over 70 mph, the transmission downshifts to give you more torque. Thus, higher RPMs.
As for the car feeling slow, that's because it has the 2.5L engine, which is somewhat underpowered for a W124. While it's not going to win any quarter miles, once you get it up to speed it's happy to stay there, as you've seen.

I love that someone is worried that an MB transmission is bad because there ISN'T any shift shock! ^_^

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RIP:
2011 E350 BlueTEC, Schwartz 81k (totaled by uninsured drunk driver)
1964 Unimog 404.1 (wrongfully towed by the city)
1994 E320, Brilliantsilber 208k (transmission shuffled off after sudden catastrophic fluid loss)
1982 300D, Silberblau under a blue repaint 256k (sold for parts)
1995 E320 Polarweiss, 131k (sold for parts)
1995 E300D Polarweiss, 287k (totaled by drunk driver)