I don't have first hand experience with this machine and perhaps you have found the best solution for the machine that you own.
Other options for you and others to consider:
(1) Unlike rotary pumps, vibe pumps have a pressure profile which gives you preinfusion, "for free." This is to say that they ramp up to full pressure much more slowly than rotary pumps, taking a number of seconds to get up to pressure. Given a choice of two machines differing only in the type of pump, if one is driven by a vibe pump it will cost less, provide the type of "forgiving" pressure profile that many people seek, and be cheaper to maintain. The downside is noise, however how many shots in a day do you make, anyway? If you can tolerate a few minutes of noise and vibration daily, a vibe pump driven machine is both more economical and will have a pressure profile that all but the most fastidious baristas will prefer.
(2) Lower doses in the portafilter = less problems with the pressure characteristics of rotary pump driven machines which lack preinfusion. There are rotary pump machines that incorporate preinfusion in their designs, however those designs can have their own pitfalls (e.g. La Marzocco's 0.6mm ruby gicleur that has a tendency to clog up). Those updosed shots that give you channeling might well flow without problems if you reduce your dose down to the range that is typically used in Italy, e.g. around 14g. Since your machine was probably designed in Italy, perhaps this explains why the manufacturer didn't think there was any problem with the machine design, because this is how they used it, over there.
(3) Assuming you want to modify your rotary pump machine, an alternate and simpler way of getting controlled preinfusion would be to put in a pressure regulator on the water input side, regulating the input pressure down to around 3 or 3.5 bar. You can then install a "delay on make" timer on the rotary pump itself, so that when you push the button to initiate a shot, the solenoid opens releasing your regulated mains pressure on the coffee cake, and only later (say 6 or 7 seconds later) does the pump itself engage. I put a DOM timer into the electronics box of my Cimbali Junior rotary machine. The DOM timer cost around $25 and the pressure regulator was relatively cheap as well. The whole modification cost well under $100 and did not require any proficiency in electronics to accomplish. I have posted extensively on this modification on HB and it should be easily found using the search function on this board. For the record, this modification was not my idea, but rather the idea of Michael Teahan, one of our resident geniuses who posts regularly here.
What, me worry?
Alfred E. Neuman, 1955