I just recently received Barayatinskiy's "Main Battle Tank T-80" book to complement Zaloga's "T-64 and T-80" and "T-80UD" books. Was reading last night that, "The turrets of earlier T-80 series had much in common with those of T-64As", and I got to thinking, how did the T-80 turret shape evolve? Looking at some of the pictures of early T-80 (page 10 & 11) turrets and you can see a decided undercut (albeit small) and is very T-64esque in general look. At what point did the "slab sided" style turret that SP designs sells for their T-80 conversions come into popular use?

Unlike T-64 (excluding first production line) or T-72 families, each new version of T-80 has UNIQUE turret in both shape and armor composition. Early T-80s had turret very similar, but not identical to ones from T-64A, T-80B had it's original, while newly built BVs had one very similar to that, but not identical... Project 219 AS received new turret designed to by Kharkov's KhBTM that was used in U series. That's the one currently depicted by SP Designs. Even so, turrets to outfit turbine powered Us are not identical to UDs (different stowage, Kontakt-5 spacing, types of wind sensors, etc). One interesting fact that is often unnoticed is that T-80U has FOUR heavy MG mount points! LOL got carried away again. Due to all this, you obviously cant use SP Designs replacement for anything other then U/UD series. SP Designs is currently upgrading existing turret sets and prepares production of newly designed T-80B conversion.
Now, if you plan on building early T-80 I might want to wait for Miniarm (although, they probably will also release T-80U conversions first) or SP Designs announced sets. Considering how good, both in surface quality(YAY) and level of detail, Sergei's TOS-1 is, new set will probably be a hit. If you fill like scratch building, 4th issue of M-Hobby for 2007 had excellent scale drawings of T-80BV by V. Malginov.

As George said, wait or scratchbuild. Tanknet is also discussing this topic, so you (other than woodstock, who started it over there too) may want to check it out here

If you want a T-80 initial, mod a T-64 turret but the hull will need some work too.

T-80B...DML, scratch, or wait.

T-80A Mod a SP Designs turret.

T-80U/UD...SP Designs.