To start with, 250 ml of 60% Ammonium Hypo solution is 150 g/l. I use 200 ml of Ammonium Thiosfulfate which is 120 g/l, and not the 20% you quote. So, I use less hypo.
Both are within the sweet spot that both Mees and Haist disclose. What they don't say, and what you have missed is that dry experiments don't agree with wet experiments but are reproducable. Wet experiments differ from dry, but vary according to developer and the treatment after the developer. So, both ways have faults. I am aware of this and I have alluded to it in my first post here. I used the dry method to formulate many Kodak bleaches, fixes and blixes and the fundamental lab work for the blix patent was done on dry strips for the sake of uniformity.
That said, lets take an example of a fix that works in 1/2 minute but requires 4 1/2 minutes to wash and another fix that works in 1 minute and washes in 1 minute. Which is the better fix? Which is the faster?
Ian is totally correct. If he can process film in 2.5 seconds then your results and mine are moot!
Now, as for some other details. The pH of a fix wrt to its formula is very important. TF-4 at pH 6 would be a different beast, and any of my super fix formulas at pH 7 is not the original at 6.5. And, urea is not thiourea. Also, urea can either accelerate or retard fixing depending on pH and thiourea alone is a slow fixing agent that also varies in fix rate as a function of pH. It is also a carcinogen.
So, the total process time, as Ian has shown us is critical as is temperature. In my second post here, I alluded to some of this by asking what the silver and hypo retention factors were at the end of your process. High levels of hypo are notorious for being difficult to wash out, and are notorious for leaving behind some silver complexes. I'm not suggesting this is the case, but I am suggesting that you test for this.
I am seeking the proper testing of your fix formula and nothing more. Yes, you see clear film. I can with a fix formula as well, but I get reticulated film from one formula and retained silver from another. All of my formulas are tested for fix rate, wash rate, and senstivity to process. The criteria are retained silver and retained hypo which relate to image stability.