Pretty much no. Sometimes it might work out, but 9 times out of 10 a GT5 tune will be useless in GT6
No. Different physics entirely between the two versions. Sure, you might get lucky here or there and get a decent handling car, but you really need a tune based on the new games physics if you want a really good handling and competitive car.
Apologies to Chameleon and CargoRat - don't mean to be argumentative but the answer is both 'yes' more than 'no' and also 'yes AND 'no', but it's going to be subjective as to whether this is something that people can understand (again - apologies, don't mean to cause offence but it's a "tuning" thing that spans all through GT5 & GT6 - I'll try to explain).
What contributes to making these matters even more confusing is what people consider as a setup that works is also subjective. Some will define this as a setup that produces the fastest possible laptime, others will define it as something that cures any issues with a car's handling i.e. understeer / oversteers.
A GT5 setup which is directly copied over, might not work owing to specific GT6 issues (camber / MR cars snap oversteer). However, if it is a camber issue and you amend the camber as per 'what works' in GT6, then potentially this would work if the setup was done a certain way.
Early on in GT5, tuning online was quite difficult to find balance, especially in situations where lower grip tyres were being used with higher PP cars i.e. 550pp sports hard. Back then, the physics meant the grip from sports soft are similiar to what sport hards are providing now.
Certain formulas for specific genres of cars (not MR/RR's) that worked very well (both in terms of lap time, but especially tyre wear) at the start of GT5, before the infamous 'mariokart' update (I think it was 2.08/2.09), can still work now. So if it was a setup done this way, then amending the camber for GT6 would make it work, albiet with reduced effect (in terms of both lap time and tyre wear) when compared to the online setups at the start of GT5.
Not sure I've explained the above very clearly, will try to simplify it for a high PP, low tyre compound situation:
Type (of setup) / potential for 'working on GT6'
Offline pre GT5 'mariokart' = NO
Online pre GT5 'mariokart) with GT5 camber = NO
Online pre GT5 'mariokart' with GT6 camber = YES
Online & offline post GT5 'mariokart' with GT5 camber = NO
Online & offline post GT5 'mariokart' with GT6 camber = YES
Applying same 'principles' for online pre GT5 'mariokart' = YES
That is being very generalistic, there are other factors that will impact this (driving style & needs for example)
Not even sure if I've managed to explain this in a way that helps to clarify the situation with setups, apologies if I've caused more confusion that explanation....