For $2500 I'd be doing a lot more diagnostics.

When the check engine light came on, what code was set?

A bad catalytic converter would limit high end performance such as acceleration and high speed. Converters can sometimes be diagnosed by kicking it. If it rattles inside, then the ceramic monolith is broken up, possibly restricting exhaust, and needs replacement. The converter can be further diagnosed by disconnecting the exhaust between engine and converter and see if the car runs normally. Though I'm not sure what the O2 sensors would do in that situation.

You need to find better service than what your are presently getting. The quote you got was from somebody who has no idea what the problem is, but figures they can try a number of things and may eventually fix it, plus their time would be covered.

If O2 sensors are still suspect, at the very least they could be changed for considerably less than $2500 to put that matter to rest before continuing with diagnostics.

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95 E320 Cabriolet, 159K