Is it a BIG upgrade having the B&C stock on the Tikka?
No. I got a B&C stock for one of my Tikkas as a gift, otherwise I never would have swapped it. No change in accuracy and it's a little (not much) heavier.
I agree with Pharmseller, the accuracy was the same after I swapped mine too. The B&C stock was more comfortable and felt better in my hands compared to the factory stock though.
I recently bought the M40 B&C stock for my heavy barrel 6.5CM Tikka.
The most accurate Tikka I've ever owned was in B&C stock. It shot good in the factory stock until the lug started moving a little. It was dented and had a little more wiggle room than I liked. Shots also walked as the barrel heated up.
I sent it to Eddie F for a bedding job. He opened the barrel channel a tad and did his magic with bedding. It was an unbelievable shooter after that.
Every Tikka I've owned has shot better once it's floated all the back to the receiver.
Would rather have a stock with the indent for an aftermarket lug . I had the B&C , the gun felt a little better and shot a little more consistently but my laminate stock tikka had a sloppy factory lug fit.