gardner
Have a look at Roberts site. Priority 1 Design, Electronics design and manufacturing
I've purchased one (and some of the chips) for an 134.2Khz project, but the chips do the same.
He has larger antenna available, freight/postage is very quick (well across the water for us), and it's based on the chips he sells as well.
He has been involved in a number of projects involving much larger antenna, and phasing them, so drop him a line outlining what you wish to do.
I agree with your summation of the Seeedstudio version...
Interestingly the 134.2 Khz tuned one from priority one was more sensitive to the 125Khz tags, than the 125Khz Seeedstudio version....by nearly three times the distance.
Mark