I already get KLM extras for having Flying Blue Platinum status. The extra Flying Blue Miles for being in Business rather than Economy is very small. As I said, the benefit is getting 30 XP rather than 10 XP for the LHR-AMS and AMS-LHR flights. Being in Business Class does avoid getting a middle seat (let's not go into the details of the seat types) since it is not possible to select seats on the KLM flights for this ticket at either MF or KL websites.
The query is: how can I see the seat selection and check the cabin sub-booking codes in Priceline - I cannot see how this?
I've never gotten anything that I wasn't supposed to from Priceline. If it says Business, I would't worry about it. But you are still given a 24 hour cancellation if it turns out not to be what you expected. Priceline isn't some shady site like eDreams or the like. Because of it's ability to have fares pushed to it (Priceline) from bookwithmatrix, many Flyertalkers have used it for booking without issue.
And regarding the benefits and luggage, that may be true in your case, but it may not be true for all people who look at this thread in the future. I was just pointing out to the other poster that the KL segment in Y is not the same as the KL segment in J, even if the seat is the same. There are benefits to non-status passengers that might make a difference. Even in your case, I'm not entirely familiar with KL's policy for status passengers, but often the luggage allowance (and therefore extra piece) is 23kg for Y and 32kg for J, so that might still be a difference, though I'm sure most people don't actually use it.