2012 Giant Roam 1

I am looking to swap out the stock stem on my hybrid for an adjustable stem to make the ride more comfortable. I tried searching for one online but could not make a decision on which one to buy. Is there a particular brand that I should get? Can anyone recommend one?

Also,
Would changing out my stock cassette improve my speed? What are my options?

Current drive train:
Shimano M391, 26/36/48
SRAM PG 950 11x34, 9-Speed

Thanks