If the clans wouldn't reduce their forces every time and assuming that a cluster have a trinary of ASF, in most conflicts the clans should easily win air supperiority. It seems the clans have a higher ratio of ASF vs mechs than most IS armies.
Attached to Mech units sure, the typical being 30-45 v/s 36-108 (I use 36 to account for DC, FRR, RCT, & CC sized units)
Where the balance comes in is the shear volume of IS Fighter Units.
The clans have their fighters attached to Warships that we can get #'s on. But outside of that it seems there isn't a lot of added free floating fighter units.
A couple stars here & there for merchant escort & then some planetary defense units but even those are likely attached to a 2nd line cluster.
The IS on the other had has HUNDREDS of more worlds in a single house than the clans do combined.
If the "typical" IS world has nothing more than a Squadron of Light ASF & 2 Squadrons of Light/Medium Convention Fighters the sheer volume starts to skyrocket.
Now those same units aren't going to get used offensively but they add to defense & harassment ability when invaded.
The IS does have plenty of free floating Aero Squadrons/Wings/Regiments.
We don't see them on FMU Rosters because those have long been stated as only applying to MECH units.
But they do show up in canon fluff, typically in Historicals over the larger wars. 4th SW Atlas, Wo39, & FC-Civil War all have plenty of notations if you read through them completely. "XYZ Aero Regiment deployed to back up this or that task force/assault."
In terms of raw #'s my guess is ratio of mechs to fighters across an entire house probably more like 50%
The only known #s in terms of Production that we have are from 3025 House Marik there they produce 2/3 fighter/mech ratio.
I can only assume that other houses are somewhat similar in ratio.
So taking into account the durability of mechs but also the less frequent/shorter combat of fighters that 50% fielded ratio can't be too far off the mark when talking about what is available for offensives.
Clan Bidding is a huge problem when it comes to Total War.
But in turn if the IS is truly pressed they have the ability to assemble a truly massive force for an offensive that the Clans can't match.
Just look at Op:Bulldog, the IS sent small portions of their military to support the DC & that force steamrolled over 1 clan.
Sure that is only 1 clan but its also only a small portion of their military.
At the End of the SL Era Kerensky had a force that would crush any single house, and could probably hold off 2 houses defensively.
But the entire might of the IS was far too much to handle. Heck IIRC that was part of the reason that Cameron formed the SL to begin with.
To let the TH get access to the far larger amount of resources that the houses & periphery had available to them.
The tech/skill advantage of the Clan/TH gave them tactical/local superiority in many battles, but in terms of logistics/strategic/attrition warfare, they are going to loose in the long run when compared to the entire volume colonized space.