Georg, I'm a beginning pattern pilot so take my advice with a grain of salt:
I got this plane (last month?) and read through this entire thread, twice. The majority of people are using OS 120 two stroke, Saito 125 four stroke, and YS 110 four stroke. I was thinking of getting an OS 120 four stroke but after reading through was talked out of it. I fly it with the 120AX, which is PLENTY of power, but after flying it this way I wouldn't want any less.

You mentioned "I am just hoping that the 115 will give enough performance to give realistic pattern performance." and if you are talking about competing, your plane will be overpowered by most of your competition. If you're just talking about basic pattern loops and rolls with not much action in your uplines, then maybe you'd be perfectly happy.

Lately I've been trying not to give advice as my experience is pretty limited, but I just maidened my Venus 40 with an OS 46AX and am disappointed by the lack of power. I'm writing this because I'd hate for you to get this heavy 8-9lb "90" size plane and be disappointed that your 115 wouldn't be enough. I've been disappointed with a couple of planes this summer and to me that's like the worst thing ever.