You're only using 64 more watts and getting 7,000-8,000 more lumen (rough estimate) depending on the specs of the bulbs. I'd say that's more efficient.

Four T12 bulbs use 160 watts and six T8's use 192 watts. The way I see it you would be adding 50% more lighting and only 20% more wattage. Definitely a good deal!

Actually, T8 bulbs are supposed to put out slightly more light than the T12's but the difference is not noticeable to the human eye.

Thanks for the feedback - it sounds like the more light the better. I may go get some more.

Here's a question for ya...... What would happen if you put T8's in a T12 fixture? Here in NY, the T8 fixtures cost almost 3 times that of the T12's. The bulbs are more too.
I'm thinking, the 2 bulb T12 fixture is balasted for 80w. 2 T8 bulbs are only 64w. What could happen?

It won't work, I've tried. The lights just flicker.

Keep checking big box stores like lowes or HD. They occasionally have sales and the T8's go for about the same as the T12's, about $10.

Boy, they sure do blink!! I bought HD's T12 2 bulb fixtures, $10 ea., and a case of T8's 56000K, $25. I couldn't get them to all light at once, blinking like crazy!! So, back to HD for T12 56000K, $15! Oh well!

Some fixtures are designed to use either T12 or T8 bulbs. The 21 fixtures I bought for my seed starting setup only cost me $1.99 each thanks to a subsidy from Southern California Edison, and I can use either T8 or T12 bulbs in them.

Totally irrelevant to the situation californication.