OBD2 readings are the tell all Chris for efficacy of your mods. It doesn't matter the specific ambient temperature its the (minimal) difference between IAT and ambient that proves the mods worth. Both parameters should be available to view via the OBD Torque app and your dongle.
FWIW you should be able to get 5% bhp improvement mid winter compared to mid summer on a normally aspirated engine assuming a cold air inlet. A very rough guide is one percent improvement for every 4°C (or 7°F) reduction in intake temp. It's not my imagination that Morton pulls harder on cold crisp mornings.
On piston engine aircraft it's doubly noticeable, you get the extra power but the wings bite on denser air with the plane seemingly wanting to leap into the air, allowing much shorter take off runs.
