As I've looked into this matter I could cut in (for a change)
The stock retainer is 17g. The Ti part will be about half if it has the same design. Now do not expect any miracles as the weight of the valve chain will not change a lot. In the 4-banger the intake valve is 110 g. The follower 42 g and the retainer 17. Thus a 50% weight saving in the retainer will save 5% weight which again will allow about 2.5% higher rpm.
As I'll use Ti valves, retainers and locks the scenario looks a bit different
However, as they are cheap, I'd go for them too.
If anyone has a source for light followers I'm all ears.