Lol manufacturers specs don't mean anything once you've changed rims and tyres... run whatever tyre pressure feels best to drive on. The reason you're feeling this is 2 reasons. First is wheel alignment, with any change of size in tyres you should get an alignment. Tyres may differ in design and work better with a different alignment, not only that but the change in size will affect alignment as well. Since its a bigger tyre the ride height would have increased which will change vehicle dynamics. Another reason is the wider rear fitment. Since the rear is bigger and wider it will make the car twitchy and basically the rear will try to swap with the fronts, and spin. Its the same reason many fwd track cars run much wider fitment on the front in relation to the rear to make the car more stable. You can band aid it with an alignment, run a bit more toe in on the rear - this may wear out your tyres quicker though.