The WAAS antennas are much more needy when it comes to "seeing" the sky...whereas the old non-WAAS antennas would work about anywhere, the likelyhoold of the WAAS antennas working really well for a long period of time while being placed under the cowl isn't good.
The Glareshield is ok for WAAS antennas, but not very good....as you need quite a bit of cable between it and the unit (at least 8') to have acceptable integrity of the WAAS system. If you stick the antenna really close to the 430/530W and don't make the cable long enough, it just won't work. If you make the cable long enough, now you just have a bunch of coax wound up under your panel, which also isn't the greatest. We've had a LOT of customers having GPS issues when upgrading their units to WAAS, and that's almost always the issue. So, we recommend placing it outside (I know, costs .5kts), but at least it'll work properly with no shadowing in any direction.
My 2 cents as usual!
Cheers,
Stein