Wow,
I am sorry to hear about your problems. I hope you did not have to pay for the engine replacement.
I have only had 2 Boxsters. And the milage was low. That being the case, I guess I never had it long enough for something to go wrong.
My first was a standard 2001 that I purchased brand new. The only thing It had was an oil leak through a spark plug. This was fixed under warranty. I sold the car with 4300 miles.
My other Boxster is a 2001 S which I purchased with 2800 miles and now have 4600. Almost time to sell

. However, knock on wood. No problems so far.
I think the most reliable Porsche I had was my 968. That was a beauty ! I put 64000 miles on it and only had to have a belt replaced and some minor items.
I am glad you bought the warranty for your car. If you decide to keep it I would extend the coverage. One thing I can think of is that your car was the first model year for the Boxster. Maybe they still had some bugs in it.
Oh, try and find info on undocumented warranty coverage. A lot of vehicles have warranty on certain compnents which go beyond the standard vehicle warranty. Most of the time you will get charged for this unless you bring it up.
JT