Have done over 100 miles now using Miller's Running in Oil and just wondering when I should be changing it. I started driving it nicely but could not resist the temptation to start having a blast.

I know it syas on the bottle that its recommended between 500 and 1000 miles but think this may be too much as its smoking and needs properly setup again. I figure the smoking is due to the running in oil and hopefully not the engine itself, fingers crossed.

 

The running in oil won't be the cause of any smoking imo, it's only mineral oil at the end of the day and i had my car run in and mapped on that oil with no ill effect. It's down to personal preferance regarding changing oil but i'd leave it in for 500 miles and drop it out and put some fresh mineral oil in for the next few hundred before mapping. You've said you've been driving it hard which is the worst thing you can do, especially if you're running some boost too. Take it for a fuelling check asap i would to make sure it's not massively overfuelling which in turn could cause borewash.