I'm a 2011 Z-6 owner who had the same problem. This is a very common complaint with the Z-6 and X-5. There is nothing wrong with your boat or engine.
Basically, the Z-6 does not have the RPS hull and is stern heavy. The 115 Opti is designed for top end performance. These factors lead to a long time to plane. Mine takes a good 8 to 10 seconds with 3 people and a full gear load. Once on plane, it runs well.
You can try redistributing weight around, or by a fin for the outboard. Really, the boat should have been rated to 150HP, and the 115HP rating is more of a pricing sacrifice.
As for the steering to the right. All engines have a natural pull. But the standard model Z-6 pulls VERY hard. This is a flaw of Nitro using a single Rack and Pinion steering system when they should have put in a dual no feedback system to begin with. My boat pulled dangerously hard to the right at plane, and the engine flopped to the right while using the trolling motor. I took it back to Nitro, and said it was unfishable that way since I constantly had to tilt the motor up. They installed a Teleflex No Feedback system and it fixed the problem. The boat is still stiff to steer, but I can take my hands off the wheel and it stays straight on course.
I STRONGLY suggest you take it back to the dealer, and make them fix it. No boat should pull so hard to one side or the other that if you lost your grip on the wheel for a second, you would be in the bank.
Anyone buying a Z-6, I strongly urge you to pony up for the hydrolic steering.