Don't listen to ill-informed rumours. On many of the engines, the 'defeat' software is not active anyway so deleting it will not affect the performance of the car at all. On others, it's possible that a different EGR system and maybe even an SCR AdBlue system will have to be added, but this will only have a marginal effect on performance and economy. You will however have to buy AdBlue, which currently sells at £16.99 for 10 litres. 10 litres normally lasts around 5,000 miles, but if the SCR system needs a lot more AdBlue, then this will become an additional expense.