I have flown my Roo's with both small and larger turbines I found the Roo2 performs great with a just 12 lb thrust class turbine, especially if this is your first jet. Top speed with the 12 lb thrust engine is approx 120-140 mph range. With a 28 lb thrust class turbine IMHO you should reduce/limit max thrust to 20-22 lb thrust, with this setting expect speeds 180-200 mph, I have been told that the Roo has been tested by manufacturer and is capable of much higher speeds than this also! 200mph with the Roo is plenty quick enough for me
I should add if you planning on flying some larger/heavier models that may require the larger thrust eg. 28lb thrust class turbine? It s still worth considering getting the larger engine and just dialing down max thrust to say 18lb max to start with until comfortable. The Roo inspires confidence quite quickly as it is very solid in the air at all speeds and helps makes the learning curve short and enjoyable