14:1 is not horribly lean. It's where it should be under normal driving conditions. The 12:1 is conservative, anything lower than that and you'll begin to lose power. 14.7:1 is the optimal air fuel ratio, 12.6:1 is the most powerful, 15.4:1 is supposedly the most economical. All that is mathematical though, so it's not necessarily perfect in real world applications. Actually, since you're not specific in your a/f ratio info, it's hard to tell where you are. 12.1:1 and 12.9:1 are all 12:1 but are pretty large in difference.
You're making 300hp N/A, and 380hp on the N20. You should never expect to gain a full 100hp from a 100shot if you're tuned reasonably. Running the N20, you should be tuned more conservative, and because of that, it will take a few ponies away.
You're looking at rwhp numbers, which are not good at showing your true power gain. 66rwhp = 80hp at the crank. To find out the gain on a 5spd, you should be dividing by .83 or so, as you'll lose about 17% of your power through the driveline.
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