Here is the link.
Does this mean than Poison Ivy does a better job of removing CO2 from the atmosphere as CO2 levels increase?
Yes.
The plant enzyme responsible for most carbon fixation, ribulose bisphospate carboxylase/oxygenase (Rubisco for short) is inefficient. While it normally catalyzes the addition of CO[sub]2[/sub] to sugar molecules, it also catalyzes the addition of oxygen to those same sugars. Oxygen and CO[sub]2[/sub] compete for the reaction. That implies that as CO[sub]2[/sub] concentrations rise, less and less oxygen will ‘fixed’ by the plant.
This is true of all plants, even C4 and CAM plants to a lesser degree, not just poison ivy.
I’m not sure what you mean by “a better job.” Increased atmospheric carbon dioxide generally results in enhanced plant growth, within limits. By incorporating atmospheric carbon dioxide in their tissues, plants provide a “sink” for excess carbon, helping to slow accumulation in the atmosphere due to the burning of fossils fuels, etc. The degree to which this will affect the overall increase of CO2 in the atmosphere is a major topic of debate. Plant growth is limited by many other factors besides CO2, such as soil nitrogen, so their capacity to sequester it may not be that great. Some researchers think that increasing temperatures as well will ultimately limit plant growth, due to the increasing need for water as temperatures rise.
Poison Ivy is a liana (woody vine) and thus is relatively better able to take advantage of increasing carbon dioxide than some other plants. Because it relies on other plants for support, it doesn’t have to use so much carbon in building wood, and thus can grow faster. Such factors may be driving the apparent increasing dominance of lianas in tropical forests.
I don’t know why increased CO2 might increase the levels of toxic compounds in Poison Ivy, but maybe it does it just because the increased availability of of resources allows it to.
Does it remove more CO2 per mass than other plants? Should I, being immune to PI, let it grow in my rural area because it removes more CO2 than say poison oak or red clover? 
There’s no indication in the article that it is better able to remove CO2 from the atmosphere than other plants, and I doubt that it does. It’s just better at converting the fixed carbon to growth than other plants. Actually, if your object is to sequester CO2 it would probably be better to plant trees, which are generally longer lived and will keep it out of the atmosphere longer by making relatively more wood than Poison Ivy does.
Darn, it is so much more fun to be good to the Earth and annoy people.
Thanks
Why don’t you plant a Upas Tree, then?
For the record, it has not been established that adding carbon dioxide is ‘bad for the earth’ anymore than removing it is ‘good for the earth’.
“The Earth” does not ‘care’ if it is covered in less ice and more liquid water anymore than it could ‘care’ if it were covered in hardy insects, tropical plants, and algae instead of mammals and birds.
The objective of concerning one’s self withing fixing carbon dioxide is purley what is in the interest of humans. Humans got here via Earthly means and whatever humans do is what is right, or ‘good’ for the earth. Ain’t no right/wrong answers.