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According to white paper, these are differences between X Performance and Z5:
Improved features:
1. Chipset: Qualcomm MSM8996 Snapdragon 820
2. Processor: 2.2 GHz / 1.6 GHz Qualcomm MSM8996 Quad Core 64-bit
3. GPU: Adreno 530
4. Front camera: 13 MP front camera with Sony Exmor RS™ for mobile
sensor (1080p)
5. Smart cleaner
6. Google calendar
7. Connectivity: Bluetooth® 4.2 wireless technology, Cast, Device Connection
Removed features:
1. X Performance is just scratch resistant, but Z5 has Scratch-resistant Chemical tempered glass + Anti-fingerprint coating
2. 4K video recording, HD video recording 720p 120fps and 1080p 60fps
3. Call settings: Slow talk, Talk equaliser, Voice enhancement
4. Small apps
5. FM Radio
6. Organiser: Calendar, Document readers/editors
7. Main camera features: 25 mm wide-angle, Face detection, Video self timer
8. Camera applications: AR Mask, Multi camera, Style Editor, Style Portrait
9. Connectivity: ANT+™, MHL 3.0 support + 5-pin support, Native USB tethering, Screencasting, Throw
10. Display: Screen video recording, Wet finger tracking
11. Hardware: IPX5 and IPX8 (waterproof), IP6X (Dust tight)
12. WIFI: QoS Admission Control
I can't find the white paper, where did you find them?
it doesn't seem that the Xperia X series support 4K video recording.
X Performance white paper still mention it can record video in H.265... so 4K could be an option. I will ask a Sony rep tomorrow at MWC
Having dinner with Sony mobile europe rep, they confirmed that there is no 4K recording for XA/X/X Performance 
If you still can, could you double check if the housing its polycarbonate or if its actual metal
Sure, I guess you're requesting for the X?
@uliwooly
Ok I doubled checked at a higher level
XA= Plastic Frame + Polycarbonate back
X= Polycarbonate frame + Metal back
X Performance = Polycarbonate frame + Brushed Metal back