If you're sure that you'll never need more than 256GB for system, applications and other internal stuff, why not? Personally, I'd never go for less than 512GB these days, but everyone's needs are different.
The T5 will be slower than the internal drive, yes, but will still be fast enough for almost any use. I use those SSDs to keep and edit my photo libraries (which are growing in size at a much faster rate than internal storage can accommodate) and they're perfect for the job.