Trick is to use ballistics. Gauss Rifles and Class 10 A/Cs don't take that many crits, but they will eat up the tonnage. The range is OK and the damage solid. You only run out of crits if you need a lot of heat sinks.
The other fun is if you use ballistics, and later swap a SFE for an XL engine, you can use a few of the tons freed up for extra ammo. Turn a regular Mech into a long-range sniper/LRM support Mech.
A comparison of XL engines vs E-S or FF armor:
SIS -> ES: you free up 5% of the Mech's tonnage (SIS = 10%, E-S = 5%)
SArmor -> F-F: free up 3-4% of the Mech's tonnage (A 100 tonner Mech uses 19.5 tons of regular armor, or 16.25 tons of F-F; 3 tons/100 = 3%)
SFE -> XL engine: Using the original Locust as an example. It mounts a 160 rated engine to be a 8/12 20 ton Mech. That engine masses 6 tons. An XL version masses 3 tons, so you are freeing up 15% of the mech's tonnage.
Out of the three options, an XL engine provides the best benefit. I'd expect to see the Clans using SFE Mechs while the factories are running, refitting to produce XL engines as they get the chance. E-S might be incorporated later, but designed for their Omni chassis. Existing manufacuring centers would make the E-S, and it would be shipped to the Inner Sphere. The Ghost Bears have an advantage here, as their infrastructure is in the IS already.
So my answer is: use SFE Mechs with Clan tech where possible, changing to XL engines as the Mechs come up for refit and XL engines are available. Ferro-fibrous armor might be used as it is a simple fix, but provides the least benefit out of the three.
To give an idea of the sheer advantage, let's look at the original
Locust:
Swapping regular armor for CF-F, I can mount 3.5 tons of CF-F, get 67 pts of protection (more than the current 64). Savings = 2.5%
Swapping SIS for E-S, I only need 1 ton instead of 2. Savings = 5%
Swapping an XL engine for SFE: Either saving 3 tons as above, or increasing the speed to 11/17 (a 220 XL engine is 5 tons, but I need 1 more ton for Gyro).
Another area to look into savings would be swapping regular Machine Guns for their Clan equivalent (half the weight of an IS MG).
Assuming I just use a XL engine, that regular Locust turns into something packing 4 ML, 4 Clan MG, and half a ton of MG ammo. Removing a single ML allows mounting 4 more MG if you want. You only get 12.5 turns of fire from the MG, but whoever is that close is not going to be happy.