A re-map will always be better if its done well but a plug in box works quite well and can be sold seperately if you sell the car. My DHSE has a PSI Power Box fitted and I'm very happy with the performance.
Mine came with the Power box fitted, I've driven one without the box and it's quite sluggish by comparison. I'ts never going to be a rocket unless you go the the extremes of Aaron beast unleashed, not recomended on an old engine in my opinion
I've had the J E Engineering remap done to my 99 dse auto - still far slower than my wife's XC90 - so I've stuck a PSI powerbox on as well - it goes like a cut cat now - but very bad overfuelling smoke on full throttle - 80mph on cruise on long runs will give circa 32-35mpg.
Transfer box chain is stretching though as a bucketload of torque means quick off the lights,etc but quite a strain.