The 1990 onwards ECU is much much smoother than the 88-89 version. The main differences are smoother off idle response, Rev limiter no longer throws you against the dashboard, and also a bit more power from 5-7000rpms. Note, I've changed an 88 JDM Si and an 89 JDM 1.5x, to 1990 onwards ECU's and noticed a huge difference. For more info read the ECU section on the main page.
88 Models had a slightly different rear suspension mounting. The inner control arm has a bigger rubber bush to create the effect of passive rear wheel steering. In effect it help the car turn into a corner.
88 European 1.6i-16 models had drum brakes on the rear, 89 onwards had rear disks.
On 88 us models there was a different clutch, the difference was in the number of splines on the input shaft of the gearbox. I dont know if this is the same for UK and JDM models.
On 90-92 models they made a slight change to the steering column, 88-89 models had four clamp type universal joints where as on 90-92 there is three clamp type universal joints and one wielded onto the steering column for greater rigidity.
90-92 models had a slightly revised joint on the bottom of the gear lever. The old style was a metal bar with two rubber O rings. The later style was a nylon sleeve that slid over the metal bar. The later type would transmit more vibration but would also reduce any slack in the linkage.
89 Onwards models had a different rubber seal on the back of the door handle. The later seal is designed to duct water away from the lock barrel
1990 onwards bodys are stiffer than the earlier models.