I don't have an answer but a similar experience. A week ago I replaced my OEM RFT all seasons with Blizzak LM-25 RFT. Immediately I could feel the straight-line stability was worse. Before it was quite good; almost like on rails. It makes me more cautious on turns but no obvious problem. So I don't get it either. Did they screw up the alignment (intentionally/accidentally)? Could it be the tires?
Mods-Etc: Quaife LSD w/VAC finned diff cover, M3 Rear SF Bushings, M3 sways & OEM Susp Front/Rear Cmpnt Kits ('cept strut brace), GC Caster/Camber Plates, AC Schnitzer M3 Sport Springs, JRZ RS1 dampers, JBD 100%, Goodrich SS brake lines, non-stggrd APEX ARC-8 18" wheels with Nitto Motivo tires (winter OEM 17" wheels and Blizzak LM-25).