My own experience with the non-Performance Base model has been in stark contrast to your experience. I'm curious as to whether your experience is indicative of the Performance Base model, or simply unique to yourself. Regarding your points:

1. I agree that it's too wobbly compared to the Surface Pro. I only use the pen while the top is detached or flipped around.

2. I also agree that it should be attachable to either side, but it's not a major inconvenience since I never use it while in laptop mode.

3. No spongey feel here. Lots of reports that it's unique to the Performance Base because there's less travel room under the keyboard.

4. I only experience lag when Battery Mode goes on the fritz and throttles the CPU too aggressively. Otherwise Windows 10 AU has been smooth as butter, and I'm running an i5 compared to your i7.

5. I was initially disappointed with the Clipboard's poor battery, but I would highly suggest turning on Battery Saver whenever you're not engaged in high performance activities. My Clipboard easily lasts about 5+ hours by itself, and 12+ hours when connected to the base. I bring my SB's charger to work, but so far I haven't had a need to use it since the battery easily lasts a whole work day while in Battery Saver mode. I'm currently at 60% and it's showing that I still have over 6 hours of battery left.