The G-R2 update pack was a group of parts released to resolve some issues that were occurring with early production Geos, that had not been picked during the testing of prototype and pre-production markers.
Geo's shoot smoothly, but there's a bit of vibration, and Geo's were found to shake their screws loose over time - once that was recognised, screws were used that have nyloc patches applied to the threads so that they don't come loose.
The reg seat seal was found to lift out of the tip of the reg piston neck too easily - this was due to the hole in the middle of the seal required by the reg's self purging feature. Air could get under the each of the seal and pop it up. This was resolved by extending a spigot through middle of the hole in the seal. This held the seal down, and allowed air to pass through a hole in the centre of the spigot.
The Prop Shaft was found to shoot loose, and modifications were carried out to prevent that.
Early Geos broke more paint than they should, and this was found to be due to the tip of the Prop Shaft being too long, and holding the ball too far in front of the bolt. This was originally done to prevent roll back, and ensure that the ball was held directly between the eyes - generally a good thing. However, the Geos bolt acceleration is so rapid, that by the time the bolt actually came in to contact with the ball, it was already moving fast enough to crack a fragile ball when it hit it - not good for tournament paint or cold weather play, or any other situation when you may have a fragile or brittle ball. Shortening the Prop tip a little kept roll back issues to a minimum, but also slowed the bolt down enough to prevent ball breakage.
So, the G-R2 update is patched screws, a new reg piston, and a new Prop Shaft. The parts were made available free to people who owned early Geos, and were fitted to all new Geos.
The ST kit is the Soft Touch kit. This was an upgrade for the Geo, the stock bolt kit for the SL91, and then stock in the Geo+. The Geo2 has a different bolt kit to the Geo or Geo+, but it incorporates the features of the ST kit for the Geo.
Geos were found to have cycling issues when shooting Reballs specifically, and when shooting at very low velocities in general. With no LPR, the Geo's cycling pressure is inextricabally linked to the pressure used to fire the ball. If you drop the HPR pressure to bring the velocity down, you reduce the amount of force available to cycle the marker. Likewise, if you drop the dwell to reduce the velocity that way, that could also cause cycling issues.
Moreover, when using Reballs, which (certainly at the time - not so much now perhaps) are smaller than paintballs, the Reball could actually block the face of the bolt. With face of the bolt sealed by the ball, a vacuum would be created as the bolt moved forwards to chamber and fire the ball. That vacuum would prevent the botl cycling correctly and cause misfires.
To resolve these problems, the ST kit included two volume restrictors that fitted around the rear of the Prop. These simply filled space in the markers firing chamber. As the Geo fires from a fixed volume of air, if you fill space in the firing chamber, then there is less air available to propel the ball up to speed. So by installing the volume restrictors, you can reduce velocity a good amount, without having turn down the HPR or reduce dwell. You can then fine tune the final velocity with the HPR without reducing cycle reliability.
The issue with Reballs was resolved by switching to a ported face bolt. There is a central hole (that the tip of the Prop Shaft passes through), with six holes around it. As the face of the bolt is relatively shallow, as the bolt moves forwards, the ball will end up resting against this central hole, but there will be a space between the ball and the outer holes. As the outer holes cannot be sealed by the ball, the vacuum is not created, and the bolt cycle will not be effected.
The third item in the ST kit is a new Can, with a single hole feeding air to and from the solenoid, rather than the six holes in the stock Geo can. This restricts how quickly air can vent out from in front of the bolt, which in turn restrictsw how quickly the bolt can move forwards. As a result the bolt cycle speed is reduced and the marker becomes smoother. Reducing the bolt speed may also help reduce the likelihood of a vacuum forming between the ball and bolt, though the ported face of the bolt should resolve that problem anyway.
While the ST kit was conceived as a solution to Reball use and low velocity play, the ST kit works perfectly with normal paint as well. Abviously you would benefit from the smoother cycle no matter what you are shooting, and the slower bolt cycle will make the bolt even less likely to crack a fragile/brittle ball. The ported face may also assist when shooting very fragile paint.