How about e91 m-sport springs? These should give you minor increase in ride height with reasonable increase in stiffness. They're intended to take the extra load that the e91 has room for (along with slightly higher rear vehicle weight).
You should currently have D4 or D5 in the rear, which are really the same spring but for slightly different applications. They are around 550lb/in.
You could try a K2 or K3 spring (again the same thing just like D4/5). These are the sport e91 spring for low spec models, and have around 570lb/in rate. Ride height should be almost unchanged, from my calcs.
Or K4 at ~590lb/in and 0~1/4" ride height gain. K5 would be ~614lb/in with similar height as the K4.
If you want similar for the front, you probably need to look at e93 335i springs. I can dig out some data on that if needs be.
YMMV with all of that. It's all data from calcs based on spring measurements from various catalogs, along with load data from BMW specs and MR from
fe1rx. In other words, I haven't tested it; use it at your own risk! (but it should be close because it makes sense and I used numbers...and I hope it helps!)
Let us know what you choose and how it goes.