After upgrading my Media Center PC running Kodi from Windows 10 1607 to Windows 10 1703, Kodi became unusable (laggy interface and laggy Live TV).
The PC is running a 5450 Graphics Card. I replaced the 5450 with a R7 240 Card and the issue persisted.
It wasn't until the latest AMD Crimson Driver was released that the issue was resolved. WDDM 2.2 Driver from what I've read.
Is there an updated WHQL Driver or Crimson Driver being released for the Legacy Cards 5xxx/6xxx/7xxx, etc to support WDDM 2.2 and thus Windows 1703?
No, legacy GPUs will not be receiving any updated drivers. The R7 240 is not a legacy GPU though, so that will continue to receive driver updates.
More information on legacy GPUs here. AMD Radeon™ Software Support for Legacy Graphics Products