The Jem'Hadar were engineered to terrorize; this might not necessarily make them particularly good soldiers. Say, their cloaks don't allow them to fight; it's difficult to understand why, unless the cloak is mainly for "secret police" type of work. That is, by being invisible, the Jem'Hadar are everywhere at once, and you can never speak freely and certain that nobody is listening behind your shoulder...
The Founders haven't encountered major opposition for 2,000 years, as per "To the Death". It might be contrary to their interests to maintain an army, even if it is chemically enslaved to them, and better to maintain a force that can merely terrorize fellow humanoid societies, with things like unopposed orbital bombardment, use of poisons and diseases, and kidnapping or other elimination of individual dissidents.
As for the Data's-eyesight-compared-to-Geordi's bit, I've been trying to find it, but it's not in "Heart of Glory" at least. Only LaForge's sight is discussed there (although Data has worked on the technique to make this sight visible to others).