If you look at the WSU Winter League scores you will find a few rifle weights posted.  The six shooters in Cody WY shot rifles as light as 12# and one as heavy as 24#.   
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You will also see that they shot considerably better than the average among all people participating.   

While my .22 offhand rifle weights in around 10+# (thus on the light side), it nonetheless balances far forward of the action.  Even though it has a back bored barrel.  I cannot imagine shooting  a rifle that balances at the action - at least not on targets.  I would not want to hunt with such a rifle either.   

From my limited experience will say that my 10# rifle that I use now is far better for me than my 14# former .22 rifle. But the weight is not the issue.  Fit is far better with the lighter rifle, and fit beats mass every time in my book.

Brent

If more weight and more weight up front is better, is it legal to add a weight to the barrel of a lighter rifle?  Or would one have to re-barrel it?

Cat here is my take on the weights.   

Different rifles work best with different weights. For example

My Hunter class NRA rules smallbore silouette rifle weighs 7 1/2 lbs. The new rule allows 8 I think but 7 1/2 was the rule when I set that rifle up. It ballances nice thumb and forefingers on the trigger gard. No hook at all allowed on the buttplate.  I shoot scores with it comprable to my full Schuetzen .22 when praticing I use a lot of cheek preasure and keep the rifle in a "pocket" formed my my sholder.

On the other side of the scale my CPA 44 1/2 fitted with it's rimfire barrel and with a Pope type stock and scope weighs over 14 lbs. It balances on the palm rest. Since the buttplate is hooked it rides between bicep and deltoid muscle, on your arm not in the "pocket" formed by your sholder. 

In the middle the Walther 10m air rifle weighs about 10 lbs  and has no hook to it's buttplate. It's completly adjustable, you can add weights on the muzzle and I have when shooting it with a palm rest.  Throws me completly off balance. Weight forward needs something Aft to keep things steerable. Lack of a hooked plate is the problem. The only way I can shoot it offhand is to mount the rifle thumb and forefingers balancing on the trigger guard.

The best way to play with balance vs weight is a "free rifle" like the Anschutz 54's you can set them any way you want weight ballance hook/flat plate. Palm rest or not, cheek piece height too. What you will find is it's not heavy or light but ballance that makes the rifle shoot offhand.

So you can see it "depends"

Boats