Is it loose strife?

I have a mounded plant with leaves similar in construction to the purple loose strife leaf. Howeve the plant is much more formed (shrubish) than any of the dozens of pictures I've found on the internet of loose strife growing in wetlands. The plant was installed in Illinois by my landscaper three or four years ago. He also installed gooseneck. Since the gooseneck was not in a controlled border it has begun to really take over. I just spent the morning clipping, derooting and hopefully beginning a successful campaign of eventually eradicating gooseneck from my garden.

However, a landscape consultant visited this weekend and said the purple loose strife plant is an innocent look alike. She even suggested splitting it for a second planting area I'm working on. She is CERTAIN it's not loose strife. She said it is a plant which is commonly confused for purple loose strife, but she couldn't name it. I have had it for over three years in a very active and potent garden with absolutely no spreading at all. How can I be sure this gorgeous plant can stay?