Well, I think I hit my personal limit. Got all elite golds for Fury, and have only two left for HD, but it's impossible.
The two are single races for Sebenco Climb and Chenghou Project - my two worst tracks. I'm not that bad at Sebenco, actually, but the AI there is just unfair with how it handles the hairpins, especially the first one where I seem to lose a second to the AI every time no matter how well I take it. As for Chenghou, I lose so much damn time going around the slanted long turn in the middle of the track. Such a shit corner - not even autopilot can get you around it efficiently.
Have tried both around 12 times, and have never even been in first once at any stage. Silver on Sebenco and Bronze on Chenghou is the best I've done, but in neither race did I threaten to win.
Oh well, it was a fun journey and an amazing, amazing game.
Sebenco is easier than you think since there's plenty of barrel rolls opportunities.
1) first corner of the lap. Pick the speed up pad and barrel roll
2) long straight part that goes a little bit down in the middle of a lap can be used for a barrel roll too. It's also the best place to use turbo
3) after the long straight, be sure to pick the second speed pad and barrel roll out of it.
4) after the S part in the end, you can barrel roll. Though this one is a bit difficult.
Remember to throw away any items you don't need and focus on try to get turbos. It helps a lot in Sebenco and Chenghou.
Chenghou is probably the worst track of HD and I agree with you that it can be a pain the ass. Basically, what I can recommend is:
1) After the tight corner in the beginning, do two barrel rolls in the jumping part.
2) Right after number 1, it's difficult to keep your ship on the ground, but try to do it as close to the left side as possible. The more you float, the more speed you lose.
3) After getting out of 2, it's one of the best places for a turbo if you got one.
4) After the long jump (don't forget to barrel roll) look for the item pads. You want to use them to refill your energy or pick a turbo.
If you still have problems, I can do a video later showing how to beat those. I think I still have to do them myself, I don't remember finishing Meltdown yet.