For various reasons, the exact optimum temperature activity for rubisco is impossible to establish with any certainty and generally, most papers discuss activity within a specific temperature range. Furthermore, oxygen and carbon dioxide levels and the presence or absence of rubisco activase can also influence the temperature - the presence of high carbon dioxide or low oxygen will influence the denaturing temperature range of the enzyme.
To complicate matters further, the temperature range for optimum rubisco activity is higher in vitro than in vivo by up to some 20 degrees C in some species and yes, optimum activity can vary between species ( the environment is a factor here).
I think you need to look at some of the learned journals to appreciate the issues concerned as it's a bit too complicated to cover in detail on AB. There's a fair amount of full text articles available via the eLibrary service which is available online from university/college libraries or larger public libraries. You'll also find some more obscure discussions on these issues in Science Direct and various biochemistry journals etc.
Incidentally, I'm sure you realise that the optimum temperature activity does not occur immediately prior to the start of decreasing rubisco activity.