for me personally, i'm with axe
i've been in cars with "tuning box's" installed, before and after, and yes there can be an appreciable difference,
but how it is obtained is very different from a specific map session with someone who knows there onions.
for me, again personally, Nigel of Angeltune is the man, and i've had him map everything mappable i've owned
over the last 5 years, all my JTD's and even my Van.
i could not be happier with either the results or the service, top man indeed.
you can have a rolling road map session of you want to pay the extra for the rolling road time, and of course the dyno operator/owner, who probably does mapping themsleves, doesn't mind you bringing your own man and laptop along,
OR, you can have a map installed according to your requirements at your work or home, as i have done in the past,
however, if you're a sceptic, you may want to know just how good that map was, and book yourself a dyno session just for a check up, which again i have done, and Nigels work was bang on the money.
if you call him, tell him Shrew told you to ring, you'll be looked after i'm sure.
all the best,