Has anybody ever installed stronger motor mounts in a [DSM]?

Lots of owners have done this. In addition to simply strengthening the existing mounts, many people found that their stock mounts were torn or broken. Others say that stiffer mounts reduce the chances of wheel hop, which is in itself a large benefit. However, stiffer motor mounts don't really contribute to more power - it simply helps a high-power DSM stay together during hard launches.

There are several vendors that offer upgraded motor mounts for DSMs. Some are complete replacements, while others are inserts that fit into the holes of the stock motor mounts.

For those interested in more do-it-yourself solutions, Tom Tharp successfully improved the motor mounts on his 1G FWD by adding homemade polyethylene inserts. He wrote a VFAQ on the subject, which you would have found if you had looked in the FAQ Locator(hint, hint).

Other people have done similar things by simply filling the stock motor mounts with silicone or urethane caulking. This is apparantly a commonplace trick on the Nissan SE-R mailing list, and has been written up by an unknown SE-R owner here.

DSM owners who have tried similar methods of strengthening the mounts include Mike O'Flaherty, Spencer Hutchings, and Mark Purney, who wrote another FAQ on the subject here.

On a different note, Colby Leonard had a solid aluminum motor mount machined for his car. Read his impressions on this mod here. Also, Colby Leaonard had a solid aluminum mount fabricated for a small cost