This paper aims at better understanding the mandibular mechanics in sabretooth and non-sabretooth taxa.
The protocol to conduct biomechanical analyses is well conceived and the large number of simulations is certainly welcome. I consider the employed sample adequate for the inference provided (but look in the comments below about some statistical analyses).
Overall, the paper is well written and the topic is of interest to the broad audience. However, I have few issues that need to be addressed before the manuscript can be fully considered.

1) I think that the introduction section should be expanded and more background and clarity about the testing hypotheses should be provided.
2) The authors did not present any strategy to account for allometric variation in their sample. Strategies mesh scaling (Dumont et al., 2009) and/or force scaling to a 2/3 power rule (see Attard et al. 2016, among others) are widely employed to account for size variation. In this sense the authors must supply analyses accounting for size before any final consideration can be made.
3) The authors should better present the statistical analyses they performed. Regressions (type, dependent and independent variables) should be better introduced in the Mat & Meth section. Also, the authors should provide some background about the importance of the variables they test, these relevant details are only shortly presented in the discussion, leaving the reader a bit confused. Nonetheless, if the authors provide a test for phylogenetic signal, why you did not perform a pgls to check whether the OLS results are due to shared ancestry or not?
Overall, the Mat & Meth section should be reframed using subheadings to clearly introduce all the analyses taken.

Other minor things.
Title
Are you sure the title is appropriate? I do not see much evidence for mosaic evolution here, rather I see a strong phenotypic convergence toward similar mechanical behaviour of the mandible. I would rephrase it.

Introduction
I think that more background should be provided and previous studies on sabertoothed carnivores should be presented in a more detailed fashion.
Line 54. Please rephrase. Maybe “..an improvement in the measured performance variables”.

Material and methods.
There was nothing about scaling protocol. How did the authors account for size? There’s no mention of force scaling too.
It would be good to show in the table which taxa is showing longer or shorter canines. Also, it would be good to provide estimates of body mass to give an idea of the size variation in the sample.

Lines 98-99-100. What are the ratios? Why are they introduced at this point? Why should they impact the biomechanical measures? Also, what kind of regressions have been performed? The authors should provide more context here.

Results
I noted that there was a weak phylogenetic signal for both Mechanical Efficiency (ME hereafter) and for Adjusted Strain Energy (ASE hereafter) at 90° gape (I guess that in the case of ASE the p-value should be 0.038 rather than 0.38, which is non-significant). Here would be interesting to map these metrics on the phylogeny to understand which closely related species are showing more similar values. This can be easily done using the function ContMap from the phytools package.
Lines 185-186: This is emblematic. You need to account for allometry.
Lines 233-237. I think there is no need of MANOVA here. Your clades are composed by a very little sample (2 in the case of Barbourofelinae) which makes the MANOVA results unreliable.

Discussion
Lines 248-263. I suggest moving this part in the intro.
Lines 357-360: I suggest removing this part as it does not add that much to the debate and, most important, you haven’t provided any test of non-adaptive factors acting on the evolution of the mandible. It is much better to focus on the many-to-one mapping which is supported by the presented data.

Kind regards.