If you want to allow a tank fire at elevations typical of howitzers you would have to increase the height and weight of the turret to a ridiculous extend.Even with the most optimum elevation (45 degree), I highly doubt it will make normal smoothbore tank guns firing at the optimal maximum range of 4km~8km to become 100 km. In fact, I think that maximum range of 8km is probably fired at close to 45 degree.
If tank guns can fire at close to normal artillery range (like the M777 Howitzer - 24 km), there would be no use for mobile artillery then. Wouldn't you think?
The muzzle velocity of a tank gun firing APFSDS rounds is above 1.6 km/sec, but take it to be 1.4 km/sec, neglect air drag and assume a flat Earth. At an elevation of 45 deg. the velocity horizontally and vertically is 1 km/sec. Taking the acceleration of gravity as 10 m/sec you will then find a flight time of 200 seconds and a range of 200 km. This shows that I'm in the right ball park.