Is it possible to safely mine bitcoin at 13.5 TH/s with hardware with 10.5 TH/s?
You’re not going to get any solid answers for what is safe and possible. Mining hardware varies a lot in what the chips can actually perform at reliably, even between individual units of the same model. 

Overclocking mining hardware in general is a fools errand, there’s a non linear relationship between the frequency and power usage. That is, by adding 20% more power you’re not going to get 20% more performance out, the efficiency is significantly degraded. When overclocking hardware runs out of specification for the cabling and voltage conversion, which significantly increases the risk of fire and damage.