Short answer is...if customs is doing their job it won't get through the boarder.
In general the KJW one you picked out is hit or miss. Some work decently well...others break down quickly. It's not a mark against the retailer...it's more to do with the manufacturer and that model. The G23 has a very, very small magazine (where the gas is held)...and so is more affected by temperature and variable in performance depending on how fast you shoot.
Searching is key to your questions. Here are some keywords that you would want to look for:
- customs (it'll return A LOT of threads...so check the FAQ's first)
- cooldown (cool down), temperature, gas pressure, propane
- XXXXX review (i.e. G23 review....or USP review)....there aren't a whole lot of "tell me all about this gun" reviews on this site, but plenty of Q&A stuff...try Googling "Justpistols" to find a link to a great review site that has lots of info on airsoft pistols.