HP 8770W DC here. Upgrades until now:
CPU: i7 3940XM
RAM: 32GB DDR3L 12800 Kingston HyperX @1866MHz/CL9 UV@1.2V
GPU: MSI GTX1070 8GB DDR5 (@1924/4400) 17’3 DreamColor IPS 10bit FullHD Display @75Hz
SSD: 512GB Samsung 860 PRO SSD + 1TB Samsung 860 EVO mSata SSD + 2TB Samsung 870 EVO SSD + 2TB HDD Toshiba L200
WiFi Modul: Intel WiFi 6 AX200
WWAN Modul: Huawei ME909s-120p v2 LTE/4G 150Mbps WWAN
GPU custom heatsink:
I don't believe there is something left to be upgraded besides eventually GTX 1080 MSi (ver. without power connector) with heavy cooling mod.