Ramy Warda :
The CPU aspect of these SoCs are pretty similar. However, the GPU in the Snapdragon 800 (Adreno 330) is faster than the Tegra 4's GPU by about 10-15%. However, the HDX runs at 1080p and the Tegra note runs at 720p. Both tablets will not have a difference in performance since the Kindle has a faster GPU but runs at double the resolution.
You should never consider higher screen resolution a detriment. There are resolution changer apps, most notably NOMone Resolution Changer, available in the Google Play Store. While so far, AOSP has not made it to the device owing to its locked bootloader, Google apps can still be sideloaded and rooting is still available.