My old MFL 3ltr MV6 had an ebay 'knock off' chip and it without doubt improved performance and economy and it did 227bhp on a set of rollers where 2 standard VX220s produced bang on factory power so would suggest fairly accurate.
I did however listen to the horror stories of the ebay chips and went out and bought an M-Tec chip.
This also produced better mpg over standard and matched the power of the ebay version - tested against previous figures on 1 mile stretch of private test track.
This also went on a RR and apparently it was producing 247bhp, which i know isn't right, on a day when most cars were recording figures a lot lower than expected, so can only estimate it was also around the 225bhp. The 247bhp figure was the one shown in the Total Vauxhall report on the RR day
For a relatively small amount of outlay there was a definate improvement in my case