So why not add 6 ounces of hops at 20 minutes ? then retain the flavour of all aroma hops.
ignoring price of course.
Yes, that's actually common to do. Well, not all at 20 minutes necessarily, but adding all of the hops for the beer in the last 20 minutes of the boil.
This is called "hopbursting" and gives much more flavor and aroma to a beer. I love it in some of my pale ales and IPAs. Here's a hops schedule for one of my beers I did as a hopbursted ale (10 gallon batch, calculated at 56 IBUs:
2.00 oz Centennial [11.90 %] - Boil 20.0 min
2.00 oz Amarillo [9.50 %] - Boil 15.0 min
2.00 oz Amarillo [9.50 %] - Boil 10.0 min
1.00 oz Centennial [11.90 %] - Boil 5.0 min
2.00 oz Amarillo [9.50 %] - Boil 0.0 min
That beer was awesome! Here's another, calculated at 60 IBUs:
1.00 oz Columbus (Tomahawk) [13.80 %] - Boil 20.0 min
2.00 oz Simcoe [11.90 %] - Boil 15.0 min
2.00 oz Amarillo [10.10 %] - Boil 10.0 min
1.00 oz Columbus (Tomahawk) [13.80 %] Boil 5.0 min
2.00 oz Amarillo [10.10 %] - Steep/Whirlpool 0.0 min
1.00 oz Simcoe [11.90 %] - Steep/Whirlpool 0.0 min
1 oz Simcoe (dryhop 5 days)
2 oz amarillo (dryhop 5 days)
It's simply another technique to have in your brewing arsenal. Back when hops were really cheap, I'd sometimes do mash hopping and first wort hopping, as well as whirlpool hops. (When I started brewing, hops were usually under 50 cents an ounce, sometimes even less).
The tricky part would be to guestimate the IBUs. Since hops do isomerize below boiling temperatures, it takes some guess work and experience to get the right balance of bittering for a hopbursted beer.