DocGreen

If you think the guys on this site know more about your injector requirements than your shop, you better find another shop.

I don't know a lot about M8's, but one thing I have noticed is that they have a higher feed injector demand, even with stock heads.

Check out Alpha Performance Engines and some of their high output builds. Not all list injector sizes but some do. I know GMR Performance in the US started selling tested, high output injectors from his shop/website pretty early on when building and tuning M8's because he found those available were not reliable and not sufficient output.

I have no idea about the LS injectors.

Can get them from Bosch or unbranded (Chinese) and anywhere in between.

Many options in the harley world and some are better than others. High performance injectors give better atomisation than others.

FuelMoto sell Bosch made to custom spec. I use HPI (probably custom spec Bosch) with my HPI throttlebody.

I build my own motors so I do my own research. Your shop would do the same.

The answers are not always in the HD Accessories catalogs. More than one way to do an upgrade.

The proof of the pudding is in the eating. 

As you say, the guys you've entrusted to do your job build & (hopefully successfuly) race their engines - go with their advice. As long as you realise, that in other respects components utilised in drag engines are not always built for longevity. 

Sure, there is no harm in getting opinions/advice from the forum (it doesn't mean you don't trust your shop), this is the spirit of the forum. You'll find a hundred who claim they build their own motors - in reality it is just a fraction, and with the ones that do, it doesn't necessarily infer any acceptable competence!

Participating 'in the spirit of the forum' requires an ability to separate the wheat from the chaff.

There are those who offer advice based on 'real world' experience, and there are 'keyboard experts', dispensing useless platitudes, who don't know one end of a spanner from the other.