I am quite surprised that someone thinks querystring based queries are faster than compiled queries.

Cliff - in your example, you will mostlikely have same performace as a compiled SP since you query is using only a single filter which may be having an index on it. So the performance may be more or less the same.

If you are getting better performance using dynamic it will most likely boil down to your statistics being out of date. DBAs are supposed to keep updating the stats or conigure auto-stats so that the compiled SP can make better decision. So the db size does not matter much if you are using only 1 index which can be easily determined. The problem more or less come when you have complicated sql which may time take to determine and best way is to have the query compiled with updated stats.

Using dynamic query is gives rise to SQL Injection - way to hack into your db and even delete your data. So you have to be very carefully and should avoid using it.