I have just come up against the same problem. I was wondering whether to set the product up as configurable or grouped product but configurable was the only one that let me enter a price so I chose that one.
Unfortunately the discount was not applied so it seems as if the system is treating it as two distinct products rather than one product with different options. I can see that there may be occasions when this behaviour is correct but I would have thought it more common that customer discounts would be applied based on the total number purchased of this product across all its variants.
I think this should be an option. The only other way I can see is to apply the discount at checkout (which is not so eye-catching) and also seems to require the use of a coupon though I could be wrong.
Yes, I think it would be helpful (at the very least) to allow Tier Pricing on a Grouped Product, with the tiers applying to the total number of items out of a group that are purchased. I did notice (for what it’s worth) that if you add Tier Pricing to an item in a Grouped Product that the tiers show up for that particular item, though not for the group as a whole...close, but not quite.
Seems to work if for example you do this by SKUusing shopping cart rules assuming there is a unique base sku for the t-shirts. In the shopping cart rules you give a percentage discount based on the number of t-shirts in the shopping cart and then apply that discount just to the t-shirts.