There is little to gain in upgrading 105 to Ultegra.
The performance gain is zero.
The durability gain is zero.
The only thing you will gain is about a pound of weightloss.
This will hardly result in any gain in speed when riding on flat ground ... it'll depend on the total weight of you and your bicycle and especially on your position ... in the case of a hybrid with semi-upright position it'll probably be less than 0.1 mph.
The only place where this small weight difference could ever benefit you is on steep inclines but even then the difference in speed will be close to 0.1 mph or maybe a tad bit more on very steep hills.

Do it for the looks if you like ... but don't expect any measurable difference.