The French driver was fifth on the final day of running, having completed 92 laps.
"We have made progress throughout the four days and in terms of pure mechanical understanding of the car, we’ve made a significant step forward compared to last year and, specifically today, we got through all our planned work programme" he said.
"I am much more confident about the new car than I was before the test and it gives me a good feeling from behind the wheel. One of the aims of the design team was to give us drivers a car with a broader operating range in terms of set-up options and I can already say they have succeeded in that point."
"There is still a lot to do, so I’m looking forward to my next session in Barcelona."
Team principal Franz Tost has set an ambitious target of sixth place for his team this year and added that the STR8 shows potential.
"We can be satisfied with these first four days of testing," he said.
"The car showed reasonable potential and reliability was not too bad, which meant we completed a respectable number of laps, first with Daniel and then with Jean-Eric."
"In contrast to this test last year, when they were rookies, both of them knew what to do and what to expect. However, the two of them had to get used to working with new race engineers and that part of the test also progressed well. There is absolutely no point in talking about car performance at this stage."
"The most important task we face now is to analyse everything we have done over the past four days in Jerez, so as to decide what our priorities will be in Barcelona in eleven days’ time, when we start the second test session."