In theory, adding the jib should reduce weather helm, as it should move the centre of effort forwards. In fact, I find the exact opposite, but this is because I only ever sail 2-up with the jib, and 1-up without, and by the time you're twin-wiring the rudder loads are much higher than when you only have one person on board, amplifying any weather helm.

Checking the forestay position is not a great way of measuring rake, as the rigging will stretch a bit over time.

As others have said, you'll be faster with the jib up to the point where you're getting overpowered. This is especially true if you get set reaching courses.

Paul