"If I wanted to specify a certain amount of bandwidth without going over, would this policy do what I am looking to do?:"
Depends what you mean by "going over". Non-LLQ bandwidth statements provide a minimum, not a cap. To insure a class doesn't use too much bandwidth you need to police or shape within the class. (As you did in your second example, although policing video often isn't a good idea and you didn't provide a minimum amount of bandwidth in that example. BTW, also can be true for LLQ, since the implicit policer activates only when there's congestion.)
"If I have the above policy and there is no congestion on the link, and there is no voice or video traffic, could the default traffic utilize the entire pipe?"
Yes. For non-LLQ classes, unless shaped or policed, they can utilize otherwise unused bandwidth. BTW, on many platforms, I believe FQ within class-default can disrupt non-LLQ class bandwidth guarantees.
[edit]
PS:
BTW, since you're marking both VoIP and Video with EF, you could also define a second LLQ class.
e.g.
policy-map P-QoS
class VOICE
priority percent 8
set ip dscp ef
Class VIDEO
priority percent 25
set ip dscp ef
class class-default
set ip precedence 0
BTW, the implicit policers for LLQ is really to protect other traffic from some kind of "run away" LLQ traffic. Normally, you don't want LLQ traffic to be dropped by the policer. This also means, unless you really see the need to multiple LLQ class policers, and since there's only one LLQ queue, you can often combine them.
e.g.
policy-map P-QoS
class VOICE_n_VIDEO
priority percent 33
set ip dscp ef
class class-default
set ip precedence 0
