The potency is subjective in my experience. 30% THC just means that 30% of all the cannabinoids in the plant are THC. But it doesn't mean there's a lot of cannabinoids.
Let's say you have 1 mg of cannabinoids in an ounce (I don't know if that's a realistic amount, but just as an example), and 30% THC, which would give you 0.3 mg of THC. However, if there was 5 mg of cannabinoids in that ounce with 10% THC, you'd end up with 0.5 mg of THC, which is actually more total THC even though it's a lower percentage. So the TOTAL cannabinoid content is REALLY important to potency but they don't even test for it.
To add another wrench, different cannabinoids can moderate the effects of THC. For example adding more CBD makes the THC much less psychoactive. There are also likely other cannabinoids that do things we don't understand, which helps explain the entourage effect.
The fact is, the more you smoke the more messed up you get. You can smoke very potent weed, but just a little, and get the same level of "messed up" as smoking a lot more weed that is less potent.
I think that's why we're going for highest potency as growers. It minimizes the amount of plant matter you need to smoke, but you can still choose your level of "stoned-ness".