Soldato
Different body panels (often lighter), additional bracing to stiffen up the chassis, solid-mounted subframes, different (lighter) interiors. The reason that BMW starting giving the M cars different model codes from the F series onwards was because so much was changed from the base car.I can't speak about the latest M3 but traditionally a different engine, a proper mechanical locking diff, previously had different gearbox (although they have the same box now but different software), completely different suspension systems, quicker steering, more sporty seats, louder exhausts, different braking system, carbon fibre propshafts/roofs/bonnets(?) and lighter alloy wheels.