1) Cheap digital amps kind of suck. They are OK for background sound or when someone doesn't really care about the sound quality to much. But they certainly are not going to please anyone that is serious about sound quality.

2) Check the specs on your project amp. They tell you what digital amp chip the Sure piece is using. A quick read on the ST chip lets you know, that you are going to get about 25-35 watts per channel, with typical HT speakers (8Ω). That 100 watt rating is peak (when the lighting is striking).
Also, you have to get a power supply to go with it. The recommended power supply is bigger than the amp board and cost as much too.

3) At that price and size = the SnapAV mini amp is smaller and already all together. Has more features. About the same power (when talking 2ch).
Russound has the new TVA2.1, which is the same as all these too. Russound is a whore brand, so anyone can buy it for dealer cost or less:
[Link: ebay.com]

[edit]: To your point of having a 5.1 system fit behind a TV. All you have to do is buy a 'Home-Theater-in-a-Box' system for about $200-$300 (throw the speakers away if you want). Slide the small receiver behind the TV (much more slim than a Marantz digital receiver). And there you go! They have about the same power per channel as those custom digital amps (25-35 watts per channel), with WAY more features for streaming, etc...
[Link: amazon.com]

I would MUCH prefer to sell a customer a 'real' system however - install & all.