Thanks for the replies and advice/comments. I am trying to learn as much as I can about bikes and their components and hope my questions aren't too stupid!
Don't see the point of replacing a 105 cassette with an ultegra one, it's just a cassette that will do the same job at the end of the day.
Perhaps
@huwsparky can help with the wheelset question
I had thought of replacing the 105 cassette (eventually when it becomes necessary) with the Ultegra (both 11 speed by the way) as the rear mech is already Ultegra. I suppose Giant put the 105 cassette on as a cost saver, just as they have used 105 for the front mech.
As for the wheels, I wouldn't expect them to make me faster but having read numerous reviews it would appear that lighter wheels can make the bike 'feel' better which is what Huwsparky, HarryTheDog and cyberknight are saying.
Thanks once again for the replies, it all starts to make sense to an old brain!
