In respect to Eibach Prokit -v- Eibach Sportline springs: Eibach Swaybars similar in size to small H&R sway bars (Eibach 26mm/23mm) are designed to work with the softer and higher Prokit Spring on soft/soft, For lower and harder Sportline Springs, swaybars are to be set on hard/hard - according to Eibach.com website.

I ran Eibach Prokit progressive rates springs + small H&R bars (26mm/22mm) on soft/soft initially and can confirm that the sway bars work well with the spring rate in the softer Eibach Prokit Spring as Eibach.com website advises.

Setting the rear H&R swaybar to hard position at times, on longer constant radius high speed corners (80kph & above), induced a bit of heave in the rear end.

Heave is when the sway bar yaw or slip overpowers the softer rear springs causing the rear end to heave up and down like pushing your foot down on the small boat in water - this reduces the fluency of the chassis during cornering, creating a bit of crudeness in the chassis ability to negotiate corners. Generally happens at higher speeds and is consistent with what Eibach state in their website.

I eliminated this heaving effect by swapping out the Eibach Prokit/Koni FSD combo for KW V3 coilovers, with higher spring rate front and rear, at the expense of losing chassis adjustability and foregiveness in the chassis tune (esp. in the wet), but gained fluency around longer higher speed corners with the stiffer KW springs, reducing the effect of the sway bar set on its harder setting.

The initial phase of heave, I believe, is twitch.

The standard Golf GTI with DCC has a bit of twitch in the chassis on longer and high speed contant radius corners, with the front and rear end twitching of chassis mid corner in creating front and rear sway bar slip against the springs/dampers. Imo the sway bars on the stock GTI are at the higher threshold of the standard OE original Golf GTI springs - this may also account for some of the restlessness & uneasiness on the highway at faster speeds, in addition to a higher OE damper to spring low speed compression/rebound rates.

The introduction of VW Driver Gear Sport springs has reduced or eliminated this twitching effect of the standard set-up (need more time in the new DG Sport set-up to be sure ... so will keep you posted in a longer-term review).

Wouldn't bother with the progressive rate strut springs after having the linear DG Sport springs ... stick with linear for best overall results imo. Progressive rate look good on paper, but are compromised on the street.

An upgrade sway bars f & r needs to take into consideration the quality of roadway driven on. The smoother the roadway - the bigger the sway bars; the rougher the roadway - the smaller the sway bar. Imo, adding sway-bars for me would be to work the tyres more effectively in all weather conditions & roadway texture, and dial out some inherent OE understeer, rather than for outright speed.

Cheers.
WJ