I have a problem when trying to shift from the smallest sprocket on the rear casette to the bigger one when on the small chainring. It tries but it seems there isn't enough tension on the cable. If I "half click" the lever it goes up. If I tension the cable ever so slightly more then I have a problem with crisp shifts down. My setup is road/MTB so quite unusual since I would like to go with a bigger casette over 34T. My crankset is Shimano Ultegra FC-6600 53/39T, rear casette is Sunrace CSM96 11-34T 9sp, rear derailleur is Shimano XT RD-M772 Shadow, SGS 9sp, brifters are Sora 9sp ST-3500, chain is Sunrace M94 9sp. I've cut my chain using small-small method. I was worried that it is a rear derailleur capacity problem but when I calculated it, it turned out that I still have a lot of room since rear derailleur's total capacity is 45T and my setup is only 37T. Recently I have checked the rear derailleur hanger with a tool and it's straight. Also the casette and chain are almost brand new, cables and housings are brand new Jagwire LEX-SL and the cable is also Jagwire polished version. But I guess it's not a friction problem since the downshifting is good. When I am on the big chainring and smallest sprocket the problem is non existent and I can shift from smallest sprocket to biger one no problem, back and forth. I heard some people mod the rear derailleur and clamp/route the cable on the opposite side of the clamp/pinch bolt (place without the groove). But I think it is applicable for 10sp shifter and 9sp derailleur combo to have a proper pull ratio. I must also mention that I changed the cage spring tension in my rear derailleur by moving the cage spring notch to the increased tension hole to improve the downshifting. Can somebody help to outline where the problem lies?
Most road drivetrain components don’t work with mountain bike components. They have different cable pull ratios and other such discrepancies. But there are exceptions. Here's great site I found that goes into detail on which road shifters go with what MTBs derailleurs: