There are plenty of theories and many folks with experience that could offer their findings... but about the only thing that -REALLY- matters is what will happen when
YOU try them.
Could be that those .356" you've been using work well for you for a reason OTHER than their diameter and the .355" that you try next work
even better.
Could be that a different brand, lot, style, or alloy of .356" bullets work like HELL when you try those.
My recommendation is two-fold.
1) Buy some of those .355" slugs and give 'em a twirl.
2) Either the new ones, or the ones you used before-- if you find a bullet and a load that runs to your satisfaction with regards to accuracy, leading, function, performance and price -- buy them by the metric ton.
I use these in 9mm:
http://www.dardascastbullets.com/mm5...ategory_Code=9
They run very well for me in .356" size. I run them in 5 different 9mm handguns and I see some small traces of lead after 250 or so rounds in some of the pistols and less in others, but not enough to bother me in any of them. They feed 100% of the time, their accuracy is terrific and I have complete confidence in these bullets and the skill that Matt puts in to producing them.
When I purchase, I buy no less than 3,000 at a time.