Personally I could build a v6 TT for low money if done right and weight less than a V8 TT, but you have to know how to do it and usually have done it before, which most have not. I pondered the idea of a V6 TT for a long time and it makes sense a on weight to hp to where that weight is in the car. In the end I decided on a V8 for the sound and feel and the cost is about the same for me as some parts could cost more on the V6, but less of them so it is a balance.
For me to decide what engine to put in there are a lot of factors.
1-skill level of the builder - Can you fabricate?
2-ease of install - ls is done and done done and done
3-What sound?
4-What hp level? 300, 400, 500, 600, 700+
5- What type of hp?NA, tq, high rpm, turbo boost fun, SC boost fun
6-engine tune capabilities
7-Are you doing it yourself or buying it done
8-Do you like to do what others do or different
9- Why did you buy your SLC?
10- Do you want low engine weight, low weight in the car, weight as far forward as possible?
11- Do you want to be unique or like others?
I think when you start asking those questions the V6TT can be a really good match for some owners/builders depending on how they use the car and their fancy. I almost did it, almost and still think it would make a stupid fast car.
934hp V6 Toyota 3mzfe
https://www.youtube.com/watch?v=XNWbZ53_CZo