At the absolute best, the Soviets in a scenario like this would advance 250 to 300 km and then grind to a halt for 6 months while their logistical train, such as it was, caught up. This pattern is repeated with every offensive they launched in World War 2. It would be unreasonable to expect anything different in this scenario. Thus, the best the Soviets could do was maybe make the Rhine and sit.
Then, it would be the West's turn to bury them in a counter offensive. Just some other numbers: The US had just over 1 million troops in Naval Construction Battalions (who are also trained as infantry). There are almost 1 million soldiers in the US in various units not shipped overseas in 1945. It isn't unreasonable to suggest that by late 1946 the US alone could have had 200 to 250 divisions in service if they really wanted or needed to.
Also, the Soviets face a real problem in that they would find themselves in a two or three front war very quickly. What happens when Sakalin Island falls (20 - 25% of their oil supply)? What happens when their Siberian troops are cut off by bombing the @$#@ out of the single rail line supporting them?
Another real possibility is a Black Sea / Iranian / Turkish invasion as well. Unlike the Allies, the Soviets have only limited ability to move their troops strategically. Look at how long it took them to move East for their invasion of Manchuria. And, that was unopposed.
In the short term the Soviets stand to make minor gains in such a war. In the long term they lose big.