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ziconater- I agree with cpach in that I wouldn't expect a significant difference in the SHIFTING between a 105 and the Tiagra. But I do have a few questions/comments.
First is what actually is the issue you are experiencing? Is it that the chain won't catch when shifting up to the large ring, or not derail off and onto the small ring? Or does the ft der do these actions well enough but you've got chain rub that you can't trim away? Does the chain jam onto both rings when shifting (chain suck of sorts)? How worn is are the parts? If the bike was clamped in a repair stand and you were shifting the ft der would it shift much better then when actually riding? How about when you pull on the cable directly (which might require some finagling if there's that damned internal cable routing)? Is there any friction in the cable route? If the cable is detatched from the der will the lever index through all it's catch points?
All this is independent in the assessment from the der's position on the frame WRT the rings or it's movement range, both of which also have to be within a narrow range of properness.
Lastly I would also ask how you're coordinating the shifting when you're riding? Do you soft pedal for the stroke or two it takes to allow the chain to lift up and over?
This issue, poor front shifting, is a common complaint that often is more the rider then the bike/parts/adjustments. But what is really going on in your case is any one's guess without in person assessment. I assume you've tried to have a shop work on this problem, having been frustrating for two years. If so did they tell you anything, make any difference, actually test ride the bike while you watched? Have you ridden with a friend how can watch what you do when you shift and then give you feed back?
"Throwing" new parts at a problem can often fix the issue when it's a hardware one (worn out, poorly adjusted, wrong compatibility) if but for the reason that the new part is starting it's install at a base line point without the "baggage" of the previous understanding. But if the shifting issue is the rider's cause then the problem will continue. Andy.