I am having WiFi issues too, and I am using a case. I had an iPad 1 that I used all over my house with a consistent 3 bars of signal. My new iPad is all over the place with strength.
I tried out speed test last night and ran it 15 times in close succession with results that were often 1:10 in difference. Ping ranged from 29ms to 349!, DL from 2678kbps to 21551kbps, and UL from 1042kbps to 10616kbps. Weirdly, best cases and worst cases for each subitem weren't the same runs -- eg, the 349 ping also had one of the fastest uploads.
Right next to my router, I am at a consistent 3 bars of strength. However, I don't accept that my new $800 device should be performing worse than my 3 year old device.
On the plus side, I don't have any heat issues.