Difference...single digit or lower in terms of overall rigidity improvements from just welding up the K. No proof, just a guess based on other reading I've done. The thing with seam welding is it is a whole approach that gets results, not a single component. Its like an apple a day keeps the doctor away doesn't apply if the rest of your diet is Coke and Twinkies. Stitch weld all the seams in through out the car and you can gain a 30% increase. This has been proven on a 1st gen Mustang.

With that said, the gussets that support the steering box and eliminate flex in that area are much higher in the improvement range and reducing this deflection may actual approach a double digit improvement. Will that be huge on the seat of the pants meter, it depends on how in tune with your car you are. If you can feel the difference in understeer half a tank of gas makes, then you'll notice the improvement in steering mount gussets.