Which Groupset?

Tyres, bar tape and saddles.

They will forever be subjective.

I was thinking about this and isn't the best way to choose a groupset to buy the lightest one you can afford?

Reduced weight has an actual performance benefit, cycling is all about power to weight ratio. No?

No.

Considering most races come down to a sprint. I'd put money on the cyclist with a heavy groupset that shifted into the correct gear, without need to reduce pedal pressure, over the one with the flaky 1900g groupset anyday.

TBH its even the same climbing. A few grams saved wont compete with being able to pop inand out of the saddle, adjusting gears accordingly. Again still with the pedals fully weighted.