Possible to have too much soffit intake venting for roof/attic

Soffit vents ensure a continuous flow of outside air on the underside of your roof.

In cold climates, this prevents two critical issues. First, from condensation on the underside of the roof from moist air in your home or from earlier in the day that has made its way into the attic. And second, from snow that falls on the roof from melting at the top, and refreezing further down creating an ice dam that causes melt water to backup into your roof and ultimately into your home.

In warm climates, the main benefit is to cool the attic space to reduce your heating costs. Some building products break down in extreme heat. Insulation in your ceiling is only slowing the transmission of heat, but when you have a 120°F attic, you'll have a lot more heat transferring through the insulation than if it is only 80°F.

That said, older homes, and those built in warmer climates are less likely to have soffit vents. Between the gable vents and ridge vents, you likely have enough ventilation for your climate (not that more wouldn't be better).

For more details on roof vents, see tester's fantastic blog post.

Maybe the links have changed, but if this is the product that you were looking at...

then yes, I have had experience with roof to wall vents... they work; but they are for the tops of roofs, where they meet a wall (like for an addition on the side of a house). So, I don't think it would be useful for your house, if I understand your question correctly. If you installed soffits (or found a way to vent without them), then you may want to use a ridge vent (kind of the same thing), but I don't really understand a way around adding/building an overhang in order to have soffits.

I would like to see a pic of your way/solution around soffits (6" vents installed equidistant from each other).