airspeed indicator problem after TT2 transponder install

I installed a TT21 transponder over the winter, and as per the
installation instructions, tapped into my ship's static line for
altitude reporting. Yesterday I checked the transponder with Orlando
ATC and they reported a good signal and proper altitude reporting. So
far, so good.

Then I noticed my airspeed indicator seemed to be reading
significantly higher than normal, and I confirmed this with a stall
series. My Ventus 2bx, which normally stalls in the 40-45kt range
depending on bank angle, now is apparently stalling in the 50-55kt
range. The ship actually feels pretty much the same as before,
although I haven't flown for 3 months so that's a bit hard to tell for
sure.

My dry flying weight is probably about 5-10lbs higher this year than
last, as I added the TT21 itself, plus a 5-lb battery to support it.
However, I'm pretty sure that by itself doesn't explain the
significantly higher apparent stall speed.

As a troubleshooting step, I removed and plugged the static line from
the airspeed indicator, so now the ASI takes it's static input from
cabin air, and everything else should be unchanged. I haven't flown
it yet to try it out, but I will today. I also have a spare airspeed
indicator that I can swap in if necessary.

Anything else I should be considering?

TIA,

Frank