On non-turbo engines backpressure is a single issue, the pressure in the exhaust system, but on a turbo engine it's actually two issues, pressure in the post-turbo exhaust system, and pre-turbine backpressure in the exhaust manifold.
A lot of people still think turbo engines 'need' backpressure or they lose low down power, but that's total crap, in fact the opposite is true- The BEST post-turbo exhaust system would be no exhaust system at all. Bigger really is better for both spoolup and power, especially directly after the turbine wheel, and it's why fitting a big downpipe to a turbo engine makes a massive difference to spool especially, much more than the rest of the system; in fact some new very high performance production cars turbo have amazingly big turbine outlet elbows for this very reason.
I'm not one for going in to the science of why things are how they are, as in the real world it usually doesn't matter, but a big part of why a turbine spools is the pressure drop between the inlet and outlet, the bigger the pressure drop, the better for spool, so having as little as possible in your post-turbo exhaust is what you want.
But post-turbine backpressure is only part of the story, pre-turbine backpressure is one of the biggest, yet totally invisible, killers of tuned turbo engines, and is also a massive reason certain engine setups don't make the power they're hoped to either.
Pre-turbine backpressure is inevitable, it's just a turbo engine thing, one of the things that spools your turbo, and a worthwhile exchange for acceptably fast spoolup for your application. Unfortunately, too small a turbine, be it the wheel, housing, or both, can cause too much backpressure, which in turn means exhaust gas struggles to leave the cylinders, huge EGTs, higher cylinder temps, diluted fuel/air mix in the cylinders, and so on, which in turns means hugely more susceptible to engine-killing det, not to mention general reliability issues and power restriction.
Some well specced, huge turbo, drag engines, actually do have lower backpressure than boost pressure, and this 'holy grail' of scenarios actually makes for an engine that makes amazing power and hugely det-resistant. Unfortunately, these sort of setups just won't spool fast enough for 95% of applications, so tend to be found on high-end drag engines only.
In the real world, most engines can be expected to have up to twice the backpressure as boost, but more than that, and some engines do have a LOT more than that due to peoples obsessions with tiny turbine housings (and a great many badly specced turbos with overly small turbine wheels compared to compressor size), which causes a lot of reliability and performance issues, and with no way to know what backpressure they've got, most don't even realise this is the main culprit!
You often find that if your pre-turbine backpressre is more than about twice what the boost pressure is (and some engine setups can be 3 times or more!), changing to a bigger turbine housing, or ideally a bigger turbine wheel, will not only drop the backpressure, but hugely increases power and reliability per psi boost, and actually not slowing your spoolup at all despite the bigger turbine; in fact in some situations it speeds it up as it was so strangled beforehand.
So yeah, BIG exhaust is good, a backpressure gauge is good to see if your turbine is strangling your engine, and don't be afraid of a bigger turbine- It may well transform your engines performance in every way...