I have been using the HPC (
http://www.hpcoatings.com.au/) TBC1 (crown) and DF1 (skirt) piston coatings for perhaps 15 years and would summarise my experiences as follows:
TBC1 crown coating - This is a good idea in theory, but I haven't experimented exhaustively. On one occasion, the coating did not prevent a chunk of piston crown from melting (due to excessive compression ratio), but perhaps I was expecting too much from it! I would like to have the time and resources to carry out some meaningful tests to determine the benefits (if any) of the coating, but I haven't
. Thus, I'm not sure whether it really makes any difference, but it does not appear to be detrimental. As the extra cost is minimal, I usually ask for the TBC1 coating at the same time as the DF1 skirt coating.
DF1 skirt coating - I have had very good results with this coating. I was tipped off about it by Greg from the TZ350 website many years ago. I can't remember the exact details, but I will tell the story as I remember it (you may still find it on the TZ350 website):
Apparently, Greg (or someone he knew) had run a tight piston-bore clearance on an old TZ with Nikasil plated bores and coated-skirt pistons. After some time on the track, the bike gradually got slower and finally ground to a halt (ostensibly due to the inadequate clearance). On stripping the top end, there was no damage to the bores whatsoever - the coated pistons had prevented actual seizure.
That story was enough for me to try the coating for myself. I first tried both the TBC1 and DF1 coatings on one of my air-cooled engines which necessarily ran quite a large piston-bore clearance. I figured that by applying the TBC1 crown coating, piston expansion would be reduced and I could employ a smaller clearance. The DF1 skirt coating was also applied as insurance against seizure. When I ran the engine with the coated piston and reduced clearance it performed well, but then began to slow down (especially when cruising at 80km/h or more). Reducing speed allowed the engine to carry on without problems, enough for me to get home and investigate. When I dismantled the top end, there was no sign of damage to the bore and only VERY SLIGHT scuffing of the skirt coating. I had the bore honed to restore the previous clearance and have had no further trouble with that engine since. From that time, I have had the DF1 coating applied to EVERY piston I install, unless the bore is electroplated and doesn't need replating (in which case a coated skirt would be too large for the bore). However, I have used the coating to take up excessive clearance in Nikasil bores and also used coated-skirt pistons when having Nikasil bores replated. For the very reasonable cost involved, I think the DF1 skirt coating is well worthwhile.
The nominal thickness of the DF1 coating is 0.025mm (0.001"). On a number of occasions on request, HPC has applied the DF1 coating in half and double thicknesses for me, to suit existing bore sizes.
HPC recommended that a coated-skirt piston should use the usual clearance to the bore, not a lesser clearance. HOWEVER, some others suggest using a reduced clearance because the coating (perhaps not HPC's DF1) is only used for running-in and will wear off. I have found that HPC's DF1 coating DOES NOT wear off, unless perhaps you consistently run an engine with inadequate piston-bore clearance. As an example, I stripped one of my engines after running for about 2500km with a coated-skirt piston - there was only very minor scuffing of the coating and no signs of other trouble.
Of course, there are other skirt coatings from other firms which may not be the same as HPC DF1. Perhaps some of those coatings ARE designed to wear off over time. I have noticed various names for these coatings, such as Graphite, Teflon, Xylan and a few others I don't remember.
HPC's DF1 attracts oil which (in theory) means that the oil film between the piston skirt and bore should never break down. I believe that true Teflon repels oil, so will behave differently. However, some suppliers call their coating "Teflon" simply because of the association with enhanced lubrication - it may not be real Teflon at all.
I'd certainly be glad to hear the experiences of anyone who has used HPC or any other skirt and/or crown coatings.
Regards,
James
James that is good info, and food for thought. 
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The skirt coatings are probably good but to me if it doesn't make more hp on the dyno I don't usually bother