Depends on who you ask. It (Steve's/Shoodaben Mountain Runner) is much better than Guhl's because it is far more refined. It matches the expectations and role of the Concours well. I have no first-hand experience comparing Steve's to Ivan's. Pretty sure nobody else on the forum does, either. So all you will get will be recommendations on what people have tried and speculation. My speculation is that Ivan's is closer to Guhl's. I give high recommendation to Shoodaben Mountain Runner.
But the results of stock vs. any of the three flashes is like night and day.
Since you don't have a clue, then why speculate? Mine(Ivan's) is buttery smooth, my Yamahas were too. Your speculation just adds to the ****-storm about flashes.
As to the OP, just go with who you want, pretty sure they're both an amazing improvement. The reason I went with Ivan's is more than a decade of using his products and flashes on other bikes. If I had never used an Ivan's product, then I too would have gone with Steve's flash.