Explain rides harder and hurts your neck? The Sienna is a Ti bike which means you will have some "road feel" ie feel some of the bumps int he road. If you were riding carbon before that, then maybe you will need some adjustment time to get use to the Ti bike. Plus the bike has a compact frame, means it's alittle squirrelier than maybe you are used to.
If you just got a fitting and some of the changes cause you to ride more agressively, just give it time, Your body will adjust. Make sure you are beding your elbows and your head is positioned appropriately. Your back and neck should be aligned.
If you don't like the road feel, try subbing out for a carbon handlebar (if the bike didn't come with one). I think the bike has carbon rear stays and fork so alot of the bumps should be erased. Worse comes to worse, take it back to the shop and ask them.
The Sienna is a good bike. Ti is never a bad choice.