it depends :
do you have a good overlap with neighbor cells or not ? if you’re in a city, with high gsm coverage, having a phone with low TIS/TRP will probably not impact the KPI.
if you’re in a rural area, designed in such a way that at the cell border you’re having “critical” radio conditions (just before making HO to neighbor), then the impact of TIS and TRP is high.
You’ll measure : more call drop rate, less ho success rate, more emergency ho rate (on low dl signal strentgh, or low ul sign str.), less tch traffic in the cell, and also :
more sdcch assign failure rate
lower average RXLEV in the cell
less power control adaptation (the average BTS power attenuation and MS power attenuation will be a bit less than before)
that’s about it…