You have to factor in how many ticks you actually get from a rejuvenation cast to get an idea of how good/bad 4t9 is. Using
Freya +3 as an example there were 202 rejuvenations cast and as expected 202 4t8 procs averaging ~20% overheal while there were only 502 ticks out of a possible 1320 leading to ~2,50 ticks/cast. Math assumes 3272sp and 18% crit at which point you'll have 2600 ticks and 2000 4t8 procs.
So given that a rejuvenation cast with 4t8 only ticks 2,50 times you could argue that 4t8 "steals" some of the tick healing, probably true, so I'll give 4t9 3 ticks to work with.
4t8 gives an extra 1600 healing/cast with overhealing factored in and rejuvenation itself gives 2,50*2600=6500 healing, a boost of 24,6%. Again, some might be "stolen" from the ticks but being instant and frontloaded outweighs that in my opinion.
4t9 is assumed to crit at 150% like all other healing spells. So we get an extra 1300 healing 18% of the time on ticks. Now on 3 ticks we'll get 3*0,18 procs or 0,54 procs/cast meaning 702 extra healing per cast or 6,4%.
Freya may not be a fair comparison due to the spikyness of damage so lets look at
Council instead where it's more sustained.
207 casts and 907 ticks gives 4,40 ticks/cast. 4t8 overhealing has increased to 30%.
4t8 now gives 1400 healing/cast while the ticks itself gives 2600*4,40=11400 healing reducing the benefit down to 12,3%.
4t9 gets no extra ticks in this scenario since the damage is so frequent that it is unlikely that 4t8 instant heal takes away a significant portion. 18%*4,40=0,79 procs/cast for a total benefit of 1029 or 9%.
While this shows 4t9 as being pretty weak remember my sample size is extremely small, only two fights. It also disregards the stat gain from using t9 instead of t8 as well as the nature of the fights in the Coliseum. However 4t8 is always there and reliable while 4t9 is random and unless we start seeing spi/crit/haste pieces in t9 it will be hard to push crit to ~25% or higher.