With different strings in the mains and crosses the tension can be used to emphasize one or the other. With a gut/poly combination increasing the gut tension and decreasing the poly will make the gut carry more of the load and will probably give more "gut feeling"
Also remember that the string bed is deformed quite a lot during impact and the tension situation will be different from the undeformed racquet. The crosses will increase their tension a little more since they are shorter. A poly will increase its tension much more than gut since it is stiffer.
Another effect to consider is that the strings also affect the racquet frame stiffness. The mains decrease the stiffness and the crosses increase it (to a lesser extent). The combined effect is that the strings makes the racquet softer. So increasing the tensions in the mains and lower the crosses will increase this effect. How large this effect is depends on the stiffness of the frame.
racquet apps for the iPhone/iPad.