With IBU conversion, you can never be sure that the result will be the same and, more importantly, perfect. In addition, you will need to find the matching feedhorn and do not forget to make all the connections watertight.
On another hand, GI-209P is a factory made LNBF (integrated feedhorn), it already has everything and works out of the box. So, I would advise you to get GI-209P, install it on your dish properly and be happy feedhunting.
No, I do not have numerical tests between GI-209P and flanged IBU, but I have feelings that results will be more or less the same.