I wanted to avoid maths but it must happen:
To know if mass is detrimental or not , we need to know how extra mass is connected with extra strength.
Let's suppose that the ratio is the same. Then for a 10% increase of muscle mass we will have 10% increase in strength.
Now we need to know another ratio. What increase in bodyfat comes together with the LBM increase. Those numbers are not steady , as a 5% bodyfat guy will have a much harder time adding pure LBM than a 15% one.
So let's get to an average example:
200lbs , 15%bodyfat , 170lbs LBM , 30lbs fat. Lets say max squat is 350lbs , so relative strength is 1.75
He increases his LBM by 10% , 187lbs. Lets say that with those 17lbs of LBM he also added 5lbs of fat, total 35.
His new body composition is : 222lbs , 15,7% bodyfat.
Now , since strength increase is also 10% , his new max squat is 385 and new relative strength 1.73
If for 10% LBM increase the strength increase is 20% , new squat is 420 , new relative strength 1.89
If he piled just 1lb of fat instead of 5 , he would be 218lbs / 14.2%bf.
Relative strength would be 1.76 for 10% strength increase and 1.92 for 20%.
Now let's say he cuts down 20lbs , 10 of them fat and 10 of them LBM.
He is now 180lbs, 160LBM , 20fat , 11% bodyfat.
His LBM loss is 6.25%
If strength loss is also 6.25% , new max squat = 330 , new relative strength = 1.83. Expected improvement as his cut was VERY sucessful.
If his strength loss is double though ( like the previous example ) , new max squat = 306 , new relative strength = 1.7
All those supposing RFD is the same at all times.
So we need to know that strength/mass increase relationship AND the nutrient partitioning profile of the subject.
I used 1:1 absolute lbs loss for cutdown and ~3:1 absolute lbs for bulk. Just seemed 'normal'. Any studies with info on those ratios ( and also the strength/mass increase ratios ) would help this discussion a lot.
Nerd rant off!