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The slow to respond almost always entails the O2 sensor is either failing, contaminated, or failed. And yes, vaccuum leaks will often cause the engine to operate in a manner which shortens the life of O2 sensors.
At 7 years old, if they are the original O2 sensors, you should feel blessed. These have been known to fail in under 2 years due to everything from dirty fuel, leaky vaccuum lines, etc. Common life spans are 3-4 years on turbo 850s.
Because your car is a turbo, and our turbos are oil cooled, and leak quite a bit of oil, which is often injested, our O2 sensors don't last nearly as long as most clean burning naturally aspirated engines. That coupled with Canadian high contaminate fuel makes O2 sensors a common problem with Canadian 850 Turbos.
However, this contamination of the O2 sensor from exhaust gas can often be cleaned, and the life of the O2 sensor extended, atleast for a moderate amount of time, i.e. 1 year. By asking you to go for a "flogging drive" your mechanic was hoping the high rate of exhaust would help clean off the O2 sensor. It obviously didn't.
However, there is a way to clean it. I'll be brief with instructions, so if you want better ones, please search for my previous posts on how to clean O2 sensors. Basically, with the car not running, remove the O2 sensor for the exhaust, and heat the shield with a blow torch till it glows orange, and keep it that hot for atleast 2-3 minutes (try to keep the entire shield glowing). Let it air cool till when you spray non-chlorinated brake cleaner into the slits, that the cleaner does not sizzle. That only takes about 1 minute. Repeat 3 times, or until the brake cleaner wash out is clear. The first time you do it, you'll be amazed at what comes out, and remember, the majority of the gunk was vaporized!!!
I did this a while back, and my O2 sensor which was setting off the lambda light every day or so, worked like new for about 14 months afterwards. After posting the ability to clean O2 sensors, and others trying it as well, they have reported similar results.
BTW, if your O2 sensor is malfunctioning because of contamination, and not because the bulb within has been damaged, your car will feel like it has new O2 sensors. I remember the amazement at the performance increase after I cleaned mine.
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