Having lived with the oil burner for a few months now, I have some updates. A few weeks ago I was doing 600 miles a week commuting - grim, but if that is where the work is, that is where I go... The 2.4 159 was an excellent place to be. On the boring M25, you could cruise at 60 (because trying to go faster was pointless) and get an indicated 60 MPG, translating into 600 miles on a tank. The other point is that motorway miles are very relaxing: in the 156 V6, 70 is about 3000 rpm, and you know all about it. With the 2.4, I am in 6th at about 1800 rpm. It pulls like a train from that level, and if you want warp speed, just drop a cog and floor it.
Come off the M25 and on to some back roads, and the diesel happily provided more oomph than I could possibly need. There are two downsides, not sure if they are a diesel issue or generic to the 159...
1) I find the gearbox, in combination with the stupendous torque of the remapped diesel some what obstructive. You've just got to accept that is not a car for the traffic light GP.
2) It is nose heavy - tight turns lead to a "based on the current laws of physics, this feels very wrong" moment - but the car makes it. I see the 159 as a motorway/A road muncher rather than a B road toy.
At 20K miles, you're going to do a lot of hours in the car, so for me the question would be whether the spider would too much of a compromise for most of the year. No idea really, I've never had a roofless car (apart from a Land Rover...).