I've heard not so great things of Guarder. Basically as I understand it they're a step up in improvement over stock clone internals but even they're not the greatest. They are good for the price though.
This might be a bit of a computer oriented analogy and not entirely correct or relational but I'll try it anyways: Lets say you have a Computer that "can" run DDR2-1066 RAM. You currently have a cheap computer (equivalent to a clone) that has DDR2-400. You have 3 options, DDR2-667 ($50), 800 ($100) and 1066 ($200). The 667 is a slight step up (Guarder FTK), the 800 is a bigger step (I don't know any brands but lets just say it's a "Modify FTK") but costs more definitely not twice the performance though, lets say 1.7X (I know you can't just give it a quantitative metric since a lot of factors come into play with varying degrees of priority to different people) the performance of the Guarder FTK, and the 1066 costs the most but gives you the biggest step in performance (For this category lets say a Prometheus FTK) BUT it's not 4x a step up in performance, lets say it's 2.5x the performance of the stock "computer".
Now take a second scenario, you have a better computer, lets say a TM or VFC. Well the internals are good enough and lets compare that with having a computer with DDR2-800 right off the bat. Sure there could be room for improvement but anything else is pretty much a downgrade except the "$200" RAM upgrade which only bumps you up slightly in performance (lets say 1.5X the performance of it's "superior stock parts").
Final note, keep in mind this is a really crappy analogy but I hope it made it kind of clear for you. And if not then just forget it.
ಠ_ಠLess QQ more Pew Pew
READY TO >> RACE