I have owned Hifis since 2008 and like you also had both the Comp and regular Hifi in 2009. I found the comp to be very efficient, aggressive and amazing performance but was quite unstable especially at the edge of the wind, for a top rider is was fantastic though! The Hifi was just a little toned down with a slightly deeper canopy and thicker LE, both had great depower though and big top ends with the Hifi having a little lower bottom end.
The 2010 Comp was a bit of a cross between the Comp and Hifi of 2009 with a canopy depth similar to the Hifi but thin LE and struts like the Comp so was stable but also aggressive and efficient, the best of both worlds, the 7 and 9m were some of the best kites I have used.
The 2011 Hifi with the 4 struts was a bit poor, it lost it cutting edge and was too spongy, I held onto my 2010 Comps!
2012 Nirvana was an improvement on the 2011 kite and back to the solid 5 strut design, the deltaish canopy gave it the power and stability but still didn't have the efficiency and performance of the 2009/10 Comps.
The Hifi X is back to what this kite is all about, higher aspect, thin LE and high performance. After using a demo kite back in September and my new 7m I can definitely say it has the solid feel and performance of the Hifi Comps from 2009/10 but is more stable. I remember having to be very careful keeping the 2010 kite in the sky when strapping into bindings, you had to keep tension on the back lines and sometimes gently keep it moving. When I demoed the Hifi X in September I purposefully rode under it to make it drop out of the sky but it drifted back which the older models would not have done as they just hindenburged on your head! It has the same big depower and windrange it has always had which is awesome for boosting and looping as you can go out super powered and go huuuge! Many a time on windy days we have been out on Hifis when guys on Fuels have been getting destroyed and giving up.
Take a look at some photos we took when we demoed it the autumn: https://www.facebook.com/photo.php?fbid ... =1&theater
Like Tack said, give one a go, if you liked your 2009 Hifis/Comps you will definitely love this!