The ST kit isn't an efficiency upgrade anyway, though some people have stated that they have seen an improvement.
The ST kit was originally intended as a solution to problems that the stock bolt kit was having when used with Reballs. Reballs were getting sucked in to the face of the bolt, which created a vacuum that tried to stop the bolt from moving forwards. That, combined with lower HPR pressure and/or dwell (to lower the velocity for use with Reballs) lead to cycling issues.
The ST kit bolt was designed to prevent the face of the bolt getting plugged by the Reballs, and the kit included firing chamber volume restrictors to allow for velocity to be reduced without making major changes to the operating pressure or dwell.
As a further addition to the ST kit, Planet also developed a new can which reduced the rate at which air in front of the bolt was vented by the solenoid. This made for a less rapid acceleration of the bolt, and hence slower bolt speeds, making the ST kit softer on paint and reducing kick.
And that in a nutshell is what the ST kit does - allows for Reball use, allows for low velocity without cycling issues, is softer on paint and kicks less. Efficiency improvements, if any, are just a bonus.
As oki has stated, the Geo doesn't kick much straight out of the box - the ST kit certainly isn't an essential item to get the Geo working well (unless you are using Reballs and/or shooting at very low velocity) - but the ST kit is a nice upgrade, and will reduce kick lower than it is already.