I have a super short stem (70mm) when I run risers, and the change in handling is barely noticeable. I have it that short from when i was recovering from a broken elbow and wanted the bars closer and higher.
90-100 mm is pretty standard on stock bikes. I'd get out a ruler or take the old one to the LBS and just do a side by side comparison. Those Thomson stems are beautiful, but you could buy 3-4 dimensions for the price and experiment.