Would Soviet bioweapons really have a large impact in the event of their use?
Potentially?
Designed bio-weapons mean that NO ONE is immune except those who have received the specific vaccine (assuming one can even be produced). There is 100% certainty that the Soviets had produced AND weaponized modified ("mutated") small pox by the early 80s, and considerable, very compelling, evidence that they had also produced other agents, including pathogens aimed at both mammals AND plants (different agents of course). That is what has leaked into the open press, hell, the Soviets had actual fatal lab incidents at least twice (with Small pox in 1971, anthrax in 1979) only the extremely isolated location of the facility and the Soviet Union's overall restriction on movement of individuals prevented a full out wildfire. God alone knows what is still being closely held, but there is evidence that the Soviets had the capacity to produce hundreds of TONS of bio-weapon agents a year (think about that one for a second, hundreds of TONS of material when a few ounces could start a pandemic that would make the Black Death look like SARS).
What's worse is that bio-weapons, by their nature, tend to be subject to on-going mutation (this is a natural part of the virus life cycle, its why you need a new flu vaccine every year, the 2017 bug isn't exactly the same as the expected 2018 bug). Once released into the open there is a very strong likelihood that any agent would change enough to jump any fire lines (i.e. vaccines) put in place. While the most likely impact would be a death toll in the mid eight figures, even in a well planned attack, there is a reasonable chance that the figure could be much higher, especially in a scenario where there has also been a nuclear exchange and basic medical and sanitary services have broken down.
You may have missed the use of jacketed/salted weapons. The increased long term "hot" fall-out from a Cobalt or Sodium jacketed/salted warheads can be used as long term (as in a century or more) area denial. While not nearly as radioactive as the immediate by-product of a regular ground burst, a warhead with Cobalt-60 built into the casing will leave a lethal signature over the spread area of the fallout cloud for generations. Did/does any of the nuclear powers possess the capacity? AFAIK no open source has made a airtight case, but the actual design requirements are very simple, frighteningly so, compared to the other steps that are necessary to produce the physics package.
These are just a few of the variables that fall under the "full exchange" heading. Real question is just how willing is one of the player's may be to to all the way to "win" (i.e. be the last ones to die on a poisoned planet). Now, that question seems insane on the face of it, mostly because it is, but attacking/responding to an attack with a few THOUSAND nuclear weapons is already off the "mad as a hatter" chart. If you are going to kill 200 million human beings, all the decisions after that are sort of moot.