Looks like reason behind this is because they chosen Steamworks for multiplayer with dedicated servers instead of their own infrastructure, although not dead and still supported progress from accounts based on old network up to 1.4 have option sync with old accont, as it seems Steam won't give any new CD-keys for MP so since being Steamworks is harder to separate simple patch from DRM-ed stuff related to multiplayer and can't bother with releasing 1.5 without multiplayer so steamworks multiplayer will be replaced later with Galaxy multiplayer infrastructure.
Performance of 1.5 have almost no difference to 1.4, only on some machines and still is resource hog that needs community fixes (E3L4 framerate wtf!), other changes are cosmetic like not losing equipment somewhere that happens rarely or doomstick heating up meals.
Losing equipment happened to me frequently. The doomstick not heating up meals is not just cosmetic and quite frustrating.
At least we're getting the patch at (unspecified date).