Even with 400 pen you could bounce from a T-50-2 or do a 0 dmg pen... Provided your calibre isn't around 150+mm.
According to WG (and I'm 90% sure they messed up their facts), bouncing is based on angle of impact (obviously) and calibre. If calibre is larger than 3x armour, it will always count as a 90 degree impact (0 degrees being parallel to the armour), at 2x it normalizes closer to 90 degrees. Otherwise it normalizes less.
Penetration is based entirely on armour (that's actual armour that the shell passes through) vs penetration value (which is a random number within the range given for the shell(?)).
Spaced armour does these calculations twice, though I believe the shell loses speed and thus penetration.
Then it checks where you hit, modules can soak up damage. Especially tracks. If I hit tracks AND pen, what would normally be a 400 dmg hit with HE will prob become around 130-150 dmg, though the tracks will likely break. AP has a tendency to be unable to do damage to more than one module, but it can damage both a module and the tank or hit both tracks.
Also, HE is a special case, but I'm not sure how it works, I can dig up info if needed, but you can prob find it on WoT wiki.
Anyway, here's what I can say from experience: Gold ammo is only useful against tanks you would normally have zero chance to pen, mostly ones without highly sloped armour. Prem shells give nothing more than penetration, so if you bounced, gold won't help.
The largest difference can be felt with derp guns that can fire AP shells too. A premium shell can do high damage and pen easily with them.
Gold or not, sticking to weakspots on meds and heavies at or above your tier and never shoot under 30 degrees (60 preferred.) Cupola's are no. 1 soft spots on most targets.
And yes, a tier 4 scout won't match a tier 5 scout, there's actually a two-tier difference between 'em (t5 scouts actually count as tier 6 in MM)