There's a problem with "Test Track will take the brunt of it". That assumption is that everyone that rides Soarin will ride Test Track instead. However, I don't think that is a valid assumption. In fact, there's a good chance that the people that are riding Soarin' also already ride Test Track.
What will likely happen is this:
- Test Track FP+ will be in higher demand (I.E. they will sell out faster).
- Because the Option of Soarin' and Test Track will be gone - people will pull other FP+, making those other FPs more likely to be used
- When the park opens, the line at Test Track will be spiked higher initially as people all rush there.
- However, the lines will only build to a certain point. Standby lines for TT may get slightly long, but again I don't know that people that were riding Soarin' will necessarily be going here - as my bet is a lot of them were already going here.
- The distribution of riders from Soarin will likely distribute fairly evenly across the other rides, between FP+ and increased Standby.
I found this data below show Operational capacity and Daily Capacity (12 hour day) for the rides at Epcot.
OHRC Daily Capacity
Soarin 1440 17280
Test Track 1080 12960
M: Space 1440 17280
SSE 2160 25920
JII 1980 23760
The Seas 1980 23760
The Land 1440 17280
Totals 11,520 138,240
Soarin accounts for 12.5% of the daily capacity of Epcot. (Note this doesn't even include Ellen's Energy Adventure or Mexico Boat Ride.) If you distribute those riders fairly evenly, that adds about 3,000 riders to each ride. Many of those will end up on the standby line.
Many of these rides have very short standby lines on normal operating days, and often don't "sell out" of FP+, and often don't ride at capacity.
Distributing riders amongst the other rides will actually have the biggest effect on the moderately popular rides. M:S, SSE and The Seas will likely see a significant effect - where you might normally see a 20 minute standby line - the increased # of FP+ plus the increased standby might give you 40-60 minute waits instead. M:S will actually likely see the most significant hit, as people who won't be able to get a Tier 1 for Test Track will go to M:S instead, causing longer FP+ lines and MUCH longer standby lines.
When Frozen opens, the biggest shift will be away from M:S, as they might leave it as a tier 1 until Soarin' opens, but the focus of FP+ selection will be on Test Track and Frozen. However, the increase capacity of Frozen will likely be eaten up by the extra people entering the park just for the Frozen experience, keeping all the standby lines longer.
That's just my opinion of course.