Performance and performance improvements are always hard to quantify in Formula 1. What we've seen so far is that the cars are 1-2 seconds per lap faster than they were in 2015, but that can be a result of many things.
Car improvements play a big part, but some of it is also down to the tyre regulations, which in effect allow the teams to nominate one-compound softer tyres than Pirelli would have done last year.
Power unit development has improved the performance throughout the grid, unless you are Toro Rosso using a 2015 Ferrari package. The team has moved from Renault, so initially there's a slight step forward, but overall there's a performance deficit to the opposition before Toro Rosso even started.