There are multiple good products on the market these days, but the one I've used in the past is the LeafProof system. WCS sells several different types of LeafProof and other systems that you can check out here
http://www.wildlifecontrolsupplies.com/removal/Gutter_protection.html.Make sure you check the landscape for potential clogging items (i.e., maple seeds from maple trees) before deciding on a specific type and then price it out for materials, removal of old, installation, and disposal to see if its a cost effective method or not.
In my case, I was able to install a painted PVC coated 1/4" hardware cloth barrier to keep the starlings and house sparrows out of the gutters but I didn't have to worry about the water flow over the guard like you've got. Based on cost you may be able to do something similar with 19 gauge PVC 1/2" wire and install it to the lip of the gutter leaving a 1/4" - 1/2" gap between the top of the wire and the guard(the 23 ga 1/4" would be too weak I think for an unsupported edge).
Also, I forgot to mention to make sure to check the gap between the guard and the lip of the gutter to see if the real issue is that there is too much space between the two (greater than 3/4" between the two materials as that is the space sparrows need to enter) of if the true problem is that the guard is too weak and they are pushing it up to get in and out of the gutters. If it is too weak, you may be able to do a simple fix by using brackets (you'd make them yourself out of matching gutter stock about 3/8 - 1/2" thick so they have a 1/4 - 1/2" lip that goes over the guard and then screwed into the gutter lip) spaced every 1 - 4 feet to prevent the guard from being bent upwards without having to do anything to it.
Finally, I'd price out the options that are feasible then make simple mock-ups to show the client and let them decide how they wanted to proceed.