Well ... here is my opinion
The Arctic Cheetah is an example of a balance issue. Is the Arctic Cheetah better than every other light mech? I'd say yes.
Anyone care to disagree?
Just look at the scores on the light leaderboard event. The Arctic Cheetah out scores EVERY other light by a significant amount. The average scores over the top 75 are 10% to 20% higher than the next best light mech and are 50% or more higher than the less competitive lights. The leaderboard event compared light mechs under comparable circumstances driven by pilots trying to get their light mech to perform at optimal levels. The numbers may not be comparable to other mech classes but they should certainly offer some insight into relative balance among lights.
On that basis, Arctic Cheetahs effectively have a built in quirk on the order of 10% to 50% over all other lights. Is that unbalanced ... in my opinion YES.
Is the Arctic Cheetah OP? Maybe since it is strictly better than every other light and is unbalanced ... does this make it OVER-powered ... depends on how you define OVER. Is the Arctic Cheetah an instant win button? Obviously not. You can kill Arctic Cheetah's just like every other mech.
In order to get BALANCE in a game ... you have to toss away your bias. A lot of folks say "Arctic Cheetah is fine ... doesn't need a nerf" ... mostly because they like playing it the way it is ... at least in part BECAUSE it is the best light mech out there. If you play ANY mech because you think/feel it is the BEST then you are implicitly saying it is unbalanced since it is better than any other.
I have my favourite mechs that I do well in, that cater to my preferred playstyles, that I enjoy playing ... CPLT-J, JM6-S, JR7-D, HBK-4G, various MAD ... but they are middle of the pack in terms of overall performance and they take some effort and work to get good results. On the other hand, a while ago I bought Stormcrows for cbills ... and that is a GREAT mech. Lots of loadouts, fast (very fast for a medium), great firepower, good hit boxes ... I can still have bad games in it ... but it is a bit easier to get decent results than my other mechs. Arctic Cheetah, Stormcrow, Timberwolf, Kodiak, Dire Wolf can all be extremely effective ... they are probably at the top of their respective mech classes still. The advantage is probably on the order of 10% to 50% (top to bottom of mechs in the weight class) ... but they are still not BALANCED against the other choices,.
The question is how far out of balance can a mech get before it is OP? Most of those I mentioned aren't really OP anymore .. but they are still unbalanced. What is an acceptable range for mech capability within a weight class? There will always be a BEST and a WORST in any category ... the balance question to PGI is really how BIG that difference should be AND what sorts of changes would be acceptable to close that gap?
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tl;dr ... Arctic Cheetah is still the best light mech ... PGI needs to come up with a balance strategy that can address clan vs IS balance, tonnage balance and weight class balance ... quirks of one sort or another can probably be used but PGI needs a unified design strategy to do so. When that happens, the relative ranking of all mechs will change.