Sid,
The tensioner that would be fitted would be a revised and better design than the original. If properly fitted you should have no issues during your period of ownership. In any event, the parts and labour should be warrantied and so you should ask your appointed workshop how long they guarantee their labour for.
As another example, the VW UK website states reference the "Replacement parts warranty. All Volkswagen Original replacement parts carry a warranty for a period of 2 years/unlimited mileage, from the date of purchase." ... and you would be wise to use genuine VW parts on an important issue such as this, regardless as to who fits the parts.
As for whether to get the revised tensioner and components work done for circa £350 or spending that money on a warranty, who are you considering getting a warranty with (check with the warranty provider before taking any warranty out that these specific parts, this specific issue and any consequential damage in the event of failure is covered)?
Then you need to decide whether you are having the tensioner work done, or buying a warranty instead or doing both.