I'll fix what i wrote above but this is the optimal approach for both haste and mastery arcane:
Bis Trinkets (I will expect you have upgraded DMC HC):
1-2/2 HC EoT
2/2 HC LotC
0-2/2 N EoT
Oh my, just look at the procs and proc rates.. LotC has 43.74% uptime for mages, Yulon has 30.35% uptime for mages.. Yu'lon(upgraded) proc is 3261 int, Cosmos(upgraded) proc is 3935 int.
Yu'lon has 1029 static int and 0,3035*3261= 989,7135 proc int, making the total int 2018,7135 int
Cosmos proc is 0,4374*3935= 1721,169
Now Cosmos has 1312 static int on top of that, so 1312/(2018,7135-1721,169)= ~4,41.
So the result. For Relic of Yu'lon to be better than Light of the Cosmos for any mage spec, int would have to be 4,41 times stronger than any combination of haste. And you can even reforge the haste to a more useful stat as fire and mastery focused arcane mage