Great article here regarding the X1/9 prototipo. It says it was faster than the stratos and stable. The fiat motor sports guy resigned when it was axed in favour of the 131 abarth...
http://www.velocetoday.com/fiat-abarth-x19-prototipo/
http://www.velocetoday.com/fiat-abarth-x19-prototipo/