In addition to looping the synch down straps around the factory rack, I run them through the rigging on my boat deck and through the bottom part of the "J bend/brackett" of the rack to get to the factory roof rack. I figure that this way if the Thule rack does give away at least it and the boat have a better chance staying with the truck longer until I can pull over. I hope this helps, overall I think the rack is really a good affordable option that requires some added attention and caution when using but overall it is well worth the money.
I use the rapid traverse foot pack and aero blade bars. The foot pack was simple to install and stays rock solid, I check and tighten periodically but they've never been close to loose. The bars are very strong and nearly silent at freeway speeds, which start at 65 in CA. I can easily transport my 2 yaks, I use a cheap j rack bought on Amazon for my smaller lighter kayak and the Yakima even keel for the big heavy one, obviously not a symmetrical setup but they both fit easily in that configuration and I don't have much other choice, my lifetime sportfisher will not fit in J racks. For toting a couple miles to local beaches it's never an issue. I don't bother with nose straps if I'm not going more than 10 miles, but I have for longer trips. In each case there's never been a problem or any movement.
The most likely causes of excessive wobble at speed on a motorway are either that you have not placed the arm clamp sufficiently close to the chainring OR you have not tightened the clamp sufficiently. Having said that, there is a further cause where the clamp can slip UP the frame IF your frame gets thinner in that direction (eg My Cervelo S3 does precisely that). In that scenario you will need to put the arm clamp slightly further away from the chainrings at a thinner part of the frame.
The covers for the the TK8 fit kit do not sit flush on the car. There is a rather substantial gap on the inside edge of the cover. This allows water to get under the trim. Thule recommended smearing a heavy layer of grease on the metal under the rail twice a year so that the metal does not rust. This is a problem. The gap is obvious and also does not look good. Thule has been aware of this problem for a couple of years and have chosen not to rectify it.
With a Thule roof rack, you’re all set to head off on your adventures and make sure you have everything you want with you. When combined with other Thule products, our roof racks are also a starting point for carrying the extra special gear that lets you live your passions – with holders for your bikes, your skis, your canoes or kayaks, and your surfboards. Not to mention extra-secure roof boxes and spacious carrier baskets for special cargos