i think you've got the wrong idea there; in almost every case, lively is great for fermentation. occasionally, brewers will *want* to underpitch and stress the yeast out for certain flavours the yeast may give, but it's rare.
the flavour killer is not how lively the yeast is, it's the temperature. warmer temperatures cause the yeast to throw out more compounds, such as esters and phenols (phenols being largely responsible for the spicy flavours from T-58, and esters generally being fruity in flavour - sometimes excessively, like a strong green apple flavour for example).
warmer temperatures will make the yeast more lively, but cause off flavours if you go too warm. on the other hand, pitching double the yeast at lower temperatures won't cause off flavours.
in short, i'd recommend 18 - 20c for the T-58; not so warm as to make it throw out overpowering flavours, but warm enough to let the yeast character develop. also, pitching 2 packs won't really alter the taste much, but it'll make it start quicker, be less prone to infection, and also be less likely to conk out before fermentation has finished.
try it on a lovely strong ale; if you're not beyond kits, try two coopers dark ale cans in one brew, made to 18-20L. otherwise, search for my "belgian brown mule" on here. it's soooo tasty at 9 months old - still in secondary, waiting for a keg
