With the 180 hp there is a change to the empty weight CG but there are other issues to consider, depending on what conversion you have.
I converted my 18A to a 180 after nearly 1500 hrs with the O-320; of which about 500 were with the thrust line change. I had always heard the discussions about the nose heavy feel and was concerned, but I wanted to clear the trees easier when leaving my cabin.
I chose the Penn Yan conversion kit.
In discussions with Mark Englerth about the thrust Line effect, he said that for what ever reason Penn Yan used about a -6 degree setting for the engine. (could have been from the original Johnson STC that they purchased). Piper used -4 for the O-320.
The TL kit is approved for the 180 so we kept it. That combination brings the engine up to about -2 degrees.
I was surprised as the cub handled nearly the same as it did with the 150. We did lighten it up with the normal alternator, cooler, starter changes but it does have the heavy prop. I think that works out to about a wash in weight changes. There is an amazing rate of climb improvement.
On approach with either engine I apply flaps as I slow down and therefore never touch the trim. With the 180 hp it is exactly the same; no need to adjust trim. It doesn't feel any more nose heavy now then what it did before.
I rarely fly super slow approaches as I am mostly on floats. This thing really performs well and it will definitely get off the ground quicker then I would with the 150 hp. It was a huge improvement on floats.
Mark shared a theory that Penn Yan installations of -6 degrees, pitch the engine down so much that the thrust is actually pulling the nose down at slower speeds giving it that "heavy feel".
He also said that the Charley Center STC uses about a -2 setting, and is thought to be a better performer then a stock Penn Yan. If you have one of these you wouldn't want to use the TL kit.
I have two cubs. A 150 hp 18A with stock wings which is really nice and flies great. The 180 hp has squared wings and long ailerons. If I sell one of them it will be the 150. This 180 18A is "hands down" my preference!
If you have a 180 hp engine that you think is really nose heavy; you may want to check into the trust line change.