The Stealth valve doesn't have the quality of appearance that the Edelbrock valve does but I've not heard of Stealth valve failures. Where the Edelbrock valves have a swirl ground finish, the valve head on the Stealth is machined and shows the cutting tool marks. The Stealth retainers and locks should be replaced immediately. Out of the box the Stealth head does not flow as well as the Edelbrock, therefore the Stealth head will require some port blending underneath the valve seat and around the short turn to equal the Edelbrock.
As to compression, by just replacing the 906 with the Stealth you will gain about .7 point of compression. So if you're running 9.3:1 you'll go to 10:1 compression and at 500hp you might go to 514hp. The stock Stealth will way outflow the stock 906 so a horsepower gain will of course be see there. It's pretty easy though, for someone who knows what they are doing, to bring a 906 with 2.14/1.81 valves, to the flow levels of the stock Stealth.