I don't believe there is at the other crossing (which couldn't really have better sight lines, unlike Ty Glas).
I've had the opportunity to travel over this line and keep an eye on the speed via GPS.
There doesn't currently seem to be a 20 mph limit on the crossing between Rhiwbina and Whitchurch.
On a train stopping at both stations as normal we went over at about 30 mph.
On a train which was late and ran non stop from Ty Glas to Coryton it managed to reach 45 a little before the crossing and didn't have to brake until well after the crossing.
Before that, after Heath we only got to 40 mph before having to brake for Ty Glas.
So I don't think an increase from 45 to 50 mph between Heath and Ty Glas will have much impact until the new trains are in operation. For now it looks as if the changes will just make things slower.
Maybe with trains with better acceleration the inceased speed on the run between Heath and Ty Glas will more than compensate for not being able to get above 20 mph between Rhiwbina and Whitchurch, though it will have more of an impact on a non stop train.