Invitation letter & business visa from own country

The BOI that I am signing up with have a WP in the works, and I will return to Oslo with an invitation letter in order to get onto a business visa. (Currently on SETV, only doing the job interviews etc)

The agents I have used in Oslo (all legit that I can see even after probing) tell me that a week should be OK, and that an invitation letter is also enough.

In the letter there will be a specific project quoted, for one of the large telcos, and the company will vouch for all business expenses.

As I will need to return to tend to my own companies from time to time anyway I will NOT care too much about having to leave, the one thing that means everything is that I will get a business visa relatively quickly - and that there will be no major errors done.

From people I hear anything from a day to several weeks for getting onto a business visa with an invitation letter, is it really that different depending on country?

Should I perhaps ditch the agents this time around, and rather use them if something really goes haywire?

My record should be absolutely clean, except having too many visa exempts - so far only two SETVs and never stayed anything close to 180 days in Thailand...