I have always believed that Macs were better for recording audio, or at least, that is what I have always been told. I have no knowledge concerning the usage of Macs. As far as the input recording level goes, if you can not get to a -18 to -6, you will chase these ACX requirement numbers, till the cows come home, and still fail. This happens a lot with Podcasters. They will buy a pretty dynamic mic for $450.00, without knowing that it takes a min of 60dbs to power it. Now they must buy the items you mentioned above, to raise their gain.
Now, if you can get your noise floor to a -70 max and your input level between a -12 and -6, talk past your dynamic mic no further the 3 inches away, use the new RMS Normalizer set to -20dbs, you will pass the ACX requirements, every single time.
As far as setting your input levels before you record with Audacity, I must be misunderstanding your statement. I can click on my Mic input level meter in Audacity, and adjust my mic input dead on, before I start to record. That is how I place it between a -6 and -12dbs.
You can find the new RMS plug in here. RMS Normalize