It is a myth that the bullet must be supersonic to be accurate. Many long range shooters shoot past the point where the bullet falls below ~1080fps, with no problem.
The following is an interesting read:
At two miles, and with a .308 Ackley (which is not quite as powerful as a 30-06).
There is also a group of shooters that are using the .303 Brit, .308, and 30-06 in Scotland, and shooting regularly at 2,200 yds (2,000 mtrs) with no problems.
It can be done with normal cartridges, and good optics - but you need a lot (I mean a LOT) of elevation, no matter what cartridge you use.
My 50BMG is set up so I have 145 moa of elevation ABOVE
the 100 yd zero, and it should be able to reach ~2,700 meters (though I have only been able to find 1,900 meters of stretch in my part of the country).