Second week in March is still the middle of winter as far as weather is concerned up on the Ontario tribs. You will typically be dealing with brutal weather and winter bank conditions. The Salmon will always have open water since it is a tailwater but on the coldest of March days, you may still be contending with slush ice, especially in the AM.
Despite the weather still being winterlike, the one thing that isn't exactly is the steelhead behavior. By the second week in March, fish will be starting to pair up and find gravel.
Fishing can be excellent, slow, or so-so. Has a lot to do with how much water is being released, and what numbers of fish are pushing in from the lake.
The " heralded spring steelhead fishing" you mentioned is most likely the post spawn/ drop back fishing that can be red hot in later April until the water gets too warm, not March, early spawn fishing.
Again, you can have an excellent trip in March, but it will be cold for sure.
If you did run into an unseasonable heat wave, the trade off would be dealing with a very high river due to snow melt and high water release form the res.
My recommendation is to go in March if you can swing it, and get back up in late April
~James
