You've not said what your existing chain is. Size and grade? Unfortunately if it's matched to a 22 lb plow, a Rocna 15 (33 lb) might demand, ideally, a commensurate upgrade for the chain. Conversely I am going to point out that, on a strict weight budget, once the chain is strong enough and long enough, all extra weight is most efficiently invested in the anchor.
If you have heavy low grade chain, you might save a lot by swapping it for stronger but lighter stuff.
Next issue, anchor sizing: this should be done with regard to the boat and the expected anchoring scenarios. I appreciate this is the point where it becomes marginal with manual retrieval, but - the inability to handle gear should necessitate a new way to handle it safely, not a down-sizing of that gear.
The Rocna recommendation for your boat is indeed the 15 (33 lb). The Rocna sizing is based on the rationale outlined here:
www.rocna.com/kb/Rocna_sizing_recommendations
You may decide it does or does not apply to you.
You would need a larger Manson knock-off than genuine Rocna to equate, and you will still have a weaker inferior product.
Stu hinted at some of these pages which may help you:
http://www.rocna.com/kb/Rode_optimizations
http://www.rocna.com/kb/Chain
amongst others.