With The Ultimate Hat and Cosmic Insight
It is possible to get 54.75% CDR on your ultimate.
And I'm having Tons of Fun. 
EDIT: My math was slightly off. It's 53.25% CDR, not 54.75%.
9 Comments
Except Ultimate Hat doesn't exceed the cap, so congratz, you've just played 45% CDR TF that has existed since S6, but this time wasting a rune slot.
It isn’t possible to hit the 54% cd on your ult because it is cap to 45%,look your ultimate cd when you hit 35% with your bonus 10% then look it when you have 45% and you will see that the cd will still be the same.
It does exceed xd I love it on lux Or Fiora with 36 cd ult xdd
It does stack...
Nasus level 3 ult with 5% CDR from Ultimate Hat and 10% CDR from Transcendence the Cooldown is 96.6 seconds.
Buy Frozen Heart, buy Trinity Force CD at 45% +5% from Ultimate Hat, Cooldown on ult is 62.7 seconds.
Fully stacked Ultimate Hat cooldown on ultimate was 56.1 seconds.
Went into training and tried it myself, so Ultimate Hat does work and stacks... Not sure if it stacks in full effect or if they are counted as a flat 60%, or maybe the 15% from Ultimate Hat is calculated on base CD before actual CD reduction (The 45%)is taken into account but it did reduce CD on ultimate by about 6 seconds in the end.
Did some basic math..
120 x .15 = 18. (Ultimate Hat)
120-18=102 (Stacked Ultimate Hat on base Cooldown before actual CDR reduction)
45% of what's left 102.
102 x .45 = 45.9 (45% CDR)
45.9 + 18 = 63.9
120 - 63.9 = 56.1 seconds
Ultimate Hat appears to apply to base cool down it is applied first, and actual CDR is applied on the amount of what's left. So it's not 60%. It's 45% of what's left after Ultimate Hat is accounted for.
Technically no caps are exceeded. If you account for Ultimate Hat and your full 45% CDR you're really only getting about 47% total reduction on the Ult.
120 x .47 = 56.4 seconds....
I don't think Ultimate Hat is worth it honestly. But then again all the runes in that slot aren't the greatest anyway.