My x-reg Citroen Synergie is completely gutless below 3,000rpm, above which it pulls nicely, but under that there's almost nothing there - really sloooow to accelerate.

I've heard of other people having lag problems traced back to something called a swirl or butterfly diaphragm (costing around £50 to replace), but our mechanic has diagnosed something wrong with the turbo, requiring a £600 recon replacement job.

Does this sound appropriate/reasonable? Would a new butterfly diaphragm help?