Here is the way I personally look at it...
My cambelt isn't due for another 30,000 miles, when it is time to change it... It makes far more sense to me to swap engines complete for a 3.0 from 2.5
Well by the time you actually change the runners and base plate, then go for a remap you will be looking at the thick end of £400 for the bits and ECU and then as many have already mentioned, there is the trade off, the normal drivability changes as a result. Then there is the worsening fuel economy... One year of spirited driving and it would have lost in mpg what it would have cost for the 3.0 conversion anyway.
So when my cambelt is due, I may consider the engine swap, do the belts/ pump etc on the 3.0 with the engine out then swap it over which is a hell of a lot easier.
If you take into account that you can get a 3.0 V6 166 donor car for as little as £200-£300, cambelt and pump (if it needs it) Many out there which have been done recently!
With all the bits you need from the donor car. Like brake calipers etc...
220 bhp+ more drivable and no trade off with lack of low down grunt for the sake of a bit more oomph at the top end that you would rarely use on the road with just a runner/ecu upgrade.
Combine that with mods like a CDA, free flowing exhaust and equal length down pipes, you would be entering GTA levels of performance on a budget and a definate Q car.
Lets face it, It would cost about that to get the cambelt/waterpump changed at an indy anyway on a 2.5 ... Logic choice if you ask me
But I am tight ars*