As the title says, I did that upgrade about 6 months ago on my Synapse.
I was stunned at how much of an improvement there was. Most of my other 'upgrades' have been meh, but the GP4000s gripped better, rode nicer, and increased my average speed by about 1mph (ish). Nothing against the Hardshell, they served a valuable purpose, but the difference in the GP4000 28s was really noticeable - even to a numbskull like me. Oh, the flat resistance has been pretty good too (only one flat since installed).
Do it!
