Its not a question of strength... that'll hold quite a weight... its a question of rigidity... how much will it deflect, vertically and horizontally, in the middle under load?

In the catalogues you will find design tables which shows how a particular type of extrusion will deflect and that will give you a clue. If you look at page 49 of the Hepco catalog they give the formulae (use the simply supported arrangement as a worst case) and page 50 the parameters for their 80x40. If you work the math it suggests a deflection of 0.00084mm in the 80mm direction (vertically) and .003mm in the 40mm direction (horizontally) for each Newton of load for a 1200mm length. (someone check these, they seem a tad low)

Your 'L' structure is slightly more complex. The vertical deflection is inversely proportional to 0.5 * w * h^3, where h is 120 and w = somewhere between 40 and 80 (I'll hazard a guess the effective value is around 55). So if a simple 40w 80h deflects X vertically, your complex beam will deflect less, roughly 25% of that. Similarly in the horizontal the ratio is roughly 30%... these are very much rough and ready, others (Ross, routercnc, et al) can do the more detailed analysis... and tell me I am wrong!

Typically the horizontal load for a wood router is due to cutting forces and 5 - 10N would be a good allowance, giving a deflection of roughly 0.003 * 30% * 10 or 0.01mm horizontally. Vertically you have the weight of the z-axis and the router/spindle - that could easily weigh 5 - 6kg or 50 - 60N, so a deflection of .0008mm * 25% * 60 or 0.012mm in the centre of a 1200mm run.

Incidentally the deflections for a piece of 80w x 120h would be .003mm horizontally and .008mm vertically, better but at the expense of added weight.