I never heard of strontium affecting branch thickness. That's usually a sign of high or low flow.
Anyway, here's a good article on stontium.
Strontium and the Reef Aquarium
From the article...
"There has never been a published study that showed exactly what happens to the health of corals (as measured via calcification rates or any other means) when strontium is absent.
There have been numerous studies that have shown what happens when strontium is raised above natural levels. In those studies, the amount of strontium incorporated rises linearly with strontium concentration up to at least 300 ppm in
Stylophora pistillata.30 It has also been shown that calcium and strontium uptake may use the same pathways, since they are both inhibited by the same organic molecules that block certain protein transporters.30 Depending on the conditions, calcification can increase on adding extra strontium, as it does with added calcium.31,32 Nevertheless, these studies say little about whether natural levels of strontium are important for normal calcification."
and...
"My recommendation is to maintain strontium in reef aquaria at something approaching natural levels. There is no evidence that strontium at these levels in detrimental to any marine organism (although it might be). There is strong scientific evidence that some organisms need strontium, albeit not the organisms that most reef keepers maintain. Finally, there is anecdotal evidence from a number of advanced aquarists that depleted strontium is detrimental to the growth of corals that many aquarists maintain."