You have a few options imo, switch to the thicker M3 front ARB if not already done so as this helps with body roll and a good mod, fit the Eibach Prokit Rear springs which wont lower the car at all in my experience (despite what Eibach say) and what others have found on here, the Fronts do drop about -10mm on Msport but rears actually lift rear slightly if on old stock springs I found, also being stiffer rate (approx. 410lb vs msport 370lb) helps reduce roll to a degree and improves turn in.
Another option is the BMW Performance springs which are slightly stiffer again but im not sure if these lower car as ive not tried them, im tempted to try them next but like you I don't want to go any lower, part numbers on the BMW springs is a bit of minefield as well, ive got some number from usa forum for 135i which will fit the 130i as well and possibly sit a little higher due to 130i being lighter, however without fitting them its hard to say, they can be bought from Cotswolds on here for around £100 the pair.
Eibachs list individual springs now as well as complete kit on the online catalogue, so in theory im sure a dealer could get just the rears.
Part numbers for 130i Prokit rear spring is F11-20-013-01-HA
Another options is to change the Koni bumpstops for BMWP versions which are only about 10mm shorter than Msport ones, this gives a slight improvement in ride quality but not as much as the Koni ones before they become active.
Food for thought