In the example mech provided, the glass cannon has a clan tech engine, giving a geometric mean of 627. With an IS XL, it is 555. Since these are averages, to compare to an existing BV scale, you are looking at 1254 v 1110. The difference in the normal BV sum is 2619 versus 2653, so because the lower defensive BV provided by the inner sphere XL engine despite only having a 34 BV difference the effect is magnified to 144. This is the whole issue with geometric means: the further you go down either F or D, the cheaper you get. Your glass cannon's counterpart is a mech with max speed and armor and no weapons, for an assault mech jumping 8 for the same BV as a light mech thanks to massive discounts by not taking weapons--such a unit would only exist because of geometric mean discounts, as normally the value of its defense doesn't get a massive discount for not having weapons.
This means your glass cannon has a close BV to the Vulture IV B, with 2649 BV. In the geometric mean, it has 1275, so it would be facing 2 or more of your glass cannons. In a duel, this might be fair--they cripple or destroy each other at the same time, though the vulture has to split fire. As part of a force though, the Glass Cannon at the edge of range or in hidden positions, despite being a comically bad mech, will still scare the pants off anyone. Thus, in standard BV, the Glass Cannon is bad and a niche design, and loses 1v1 to the Vulture. In *root scaling, it gets a massive BV discount making it probably the best ambusher in the game, and can beat the Vulture in a straight fight some of the time.