A Metro expert suggested it would be perhaps a 30 minute job. It took me maybe 2 hours. It'd take me 30 minutes to do again though
1. I cut my timing belt cover with a dremel so I could remove just the top portion (the part that covers the cam sprocket). If you don't do this, you have to remove both the water pump pulley and the crank pulley to get the cover off (careful you don't cut the timing belt too!).
2. I was prepared for the challenge of getting the valve cover off. There are some spacer/gasket-y things under the nuts. They need to be spun off or the cover won't budge.
3. I had to go to a friend's house to use his vice (to hold the cam sprocket so I could get the bolt out and back in, tight enough). Need to buy a vice. Can't believe I've gotten away without one for so long.
Having said that...
WOW! I like it. 
- I was worried I wouldn't be able to detect a difference, or it would so small that I wouldn't be sure if I was just imagining it.
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- But the car definitely has more low end cojones. (Maybe more accurate to say it has "a cojone" now, because it had none before and I'm still not going to sign up for any tractor pull competitions)
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- I can shift at least 5 km/h sooner than I used to, and the engine just pulls smooth smooth smooth.
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- I'll have to back off the timing though. I was hearing some ping, and on my test drive I couldn't open the throttle as much as I usually do during P&G. But I was told in advance to expect this - since the cam effectively raises the cylinder pressure, uncontrolled combustion is more likely when you tune it near the edge (timing advanced beyond stock).
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- There's a tiny amount of lifter tap that wasn't there before, but hey, what could I expect from swapping out a cam with 12,000 kms on it for one from a car that is at least 4 years older, with who knows how many more miles on it (obviously enough that it was ready for the junk yard).
Anyway - on the test run, even without being able to use as much throttle as I would have liked, I was able to pull off a 84.8 mpg (US) P&G round trip - 9.8 miles, 1 hr EBH assisted start, 3C temp.
So it certainly hasn't hurt!
In fact, I think it's safe to say it's a perfect complement to the taller tranny. Like peanut butter and jelly!