So I recently found I can add more tension to my lower pullys on my road derailleur by removing the tension spring and moving it one space round into a secondry pre drilled hole, fairly simple to do but i got thinking …..would I be able to drill a 3rd hole to add even more tension ? Has anyone tried this? And what would the downsides be to too much tension ? I presume wear would be the major one ?

The question is; why?

Hmmm, yup Why?

Are you experiencing shifting issues?
Or
Drops?

If im totally honest im pre empting chain drops/ rattling about, I have a clutch mech on my mtb an I like the way it eliminates all the rattling about down hill and when the mech gets cloged up with grass it allows me to carry on riding for longer until I can find somewhere to clear it all out , curiosity led me to asking

you could end up with the end of the spring coming out of the new hole as it is under too much tension – then you lose the rear mech springiness altogether. Happened to me with an unmodified rear mech.

I would be more tempted to get a narrow/wide chainring and go 1×10, that way the chain is shorter and less likely to rattle.
(It’s what I have done and with the shorter chain I’ve now done 4 races and not dropped a chain)