I bought my 2002 S80 18 months ago and was impressed with the performance but because I tow a 23cwt caravan I decided to get it chipped(trading my previous car chip and getting a discount). It was from TD Performance Tuning.......
www.tdperformance.co.uk
I takes it from 163bhp to about 195bhp. The difference was tremendous....even towing a caravan, the traction control struggles to cope when you pull away quick into a roundabout. I tend to use cruise alot and when coming out of roundabouts, I press resume (to take me back to 60mph) and often find that I am catching up rapidly on solo cars.
The mpg, I think is slightly better....possibly about 2mpg but towing I am now averaging 37 and get from Calais to Newcastle on French Deisel!!(400 miles).
If you are used to a Petrol, like I was....previously having a 24v Scorpio Cosworth, then getting a deisel is a step down but with the chip it is almost as good.
Is it worth £300+, only you can decide that. I dont know of any disadvantages altho at 96K miles I am having to shell out on new refurbished injectors costing £1000, but I understand that there was a problem with the 2002/3 D5's an Volvo did help some within the warranty period.
Hope this helps
Geoff