The speeds and feeds recommended are NOT a pipe dream. I run deep wells with Helical Solutions and Harvey all the time.
The only way I would ever consider running 3" deep with a 3/8 diameter cutter is with a reduced shank long reach solution such as this one Helical Tool
I did 3.5" with a YG1 reduced shank and it was a beast...I think I was going 300IPM. I did 2" with a 3/8 Helical recently running at 180IPM as well. Only after all the rough machining is done do I go back in with a full flute finisher. Even then I might not actually do it...what I mihgt do is pick up a long flute reduced shank like this one https://www.mscdirect.com/product/details/78088374
It has a 1" LOC so I could finish in 3 steps and then I might come back with a 3" LOC do to a final blending finish.
It is all about doing this in steps when machining incredibly deep...anything beyond 5xD is going to require some thoughtful process development to get the optimal result.
For context, running 20xD on a 1/8" diameter end mill no longer scares me...I am used to the parameters I need to stick to and it usually works with great results. Trochoidal strategies really shine in these situations. Last week I ran 10xD on a 1/16" cutter in 316 stainless steel. Right tool for the job is necessary.