I started a tangent earlier in the thread about the value of the Bren to the US because the infantry were forced to rely on artillery more than they arguably should have to make up for the deficit in infantry firepower, which limited their effectiveness; my support for that was the lower German casualty rates than the Allies in the major battles of 1944, lower even than US official figures, despite overwhelming applications for artillery fire among other support, which then turned into a major tangent that ended up bringing in a mod to tell us to stick to the main topic.
Not sure if you saw Calbear's post, but he doesn't want any more derailment of the thread, so I'm going to have to leave the bulk of your post unanswered.
Since this part is on topic I will respond.
In semi-autos sure, but ITTL with the Bren they would still have Garands their other weapons, so it would be a substantial firepower enhancement. I get into the reasons below in more detail, but the Bren was able to produce much more and sustained automatic fire, which is how a squad achieves firepower superiority in battle. The Bren, besides the larger, more easily reloaded magazine, also had a QC barrel to keep the fire up while the BAR had to take a 90 second pause after IIRC 3 magazines or risk burning out the barrel, and had a higher ROF so the volume of sustained effective fire was substantially larger than the BAR or even potentially 2 BARs could generate, while the Bren was still less expensive than 1 BAR and no heavier. So you could equip a US squad with 2 Brens by or before the time that 2 BARs became standard issue in a US squad. 2 Brens then would be able to produce an effective fire rate equivalent to 3-4 BARs and require no more people compared to the BAR per US doctrine.
In a 12 man US squad if they divided into fire teams with one being a Bren group of 2 Brens to support the rest of the squad or two equal sized 6 man fire teams each with a Bren you'll have a much more effective squad than one with 2 or 3 BARs. They'd easily produce more than enough firepower to overpower an MG42 squad and still be somewhat competitive with a 2x MG42 squad, which did appear towards the end of the war. That's assuming there isn't a platoon level Bren group instead of squad LMGs. Or supplementing the squad MGs.
The nice part about the Bren, other than being superior to the BAR, is that it can also replace the weapons platoon M1919s as well, which means you can really have a lot of firepower for a US platoon by having more, cheaper Brens than BARs+M1919s.
Per this by 1944 there wasn't a platoon MG, but the infantry companies have a weapons platoon. For the cost of two M1919s they could have fielded three Brens instead and had only a 3 man team each instead of a 4 man M1919 team. Even without the belt fed weapons the extra Bren could increase overall firepower and be much lighter and handier for the team to move around while allowing for extra ammo to be carried from saving 4kg per MG.
The last part is related to the point of the Bren being adopted by the US, namely in that a machine gun with a higher rate of sustained fire will be the best way to achieve fire superiority in an infantry firefight. Personally I've been partial to the Bren as a LMG over every other WW2 LMG, but recently I've started to come around to the idea behind the MG42, despite it's disadvantages in weight, ammo consumption, and need for multiple 1kg barrels. Namely that the ROF does suppress better than any other MG of the period that isn't a .50 cal or higher because of the weight of fire and the psychological impact of that, which I came across some related research about by the British in the 1980s. Winning infantry engagements is mostly about being able to achieve firepower superiority over the foe and having a weapon that can beat any other infantry weapon in volume of fire will suppress better and through that create the conditions to win the engagement. That doesn't even get into firing ratios and the reasons behind that, which are also highly relevant. If you're interested I can post the paper with some commentary.
Now rather than this being a "MG42 so awesome" post, the above concept is more to illustrate the advantage of the Bren over the BAR in that the Bren had a larger magazine that was replaced more quickly so could produce a greater volume of fire, could sustain fire much longer thanks to the QC barrel, and had a cyclic rate as well. All that adds up to making the Bren a more effectively base of fire than even two BARs, while being cheaper and no heavier.