First, the Rotax on a T-51 is hanging way out in the nose (and has a gearbox BTW so it's revving super fast to get the good power/weight it has) Meanwhile if you look at an LS install on a T-51 that engine is buried back into the firewall. They share the same name, but a hotrod T-51 nowadays is not the same aircraft as one that's been rigged for LSA with the 912. Different landing gear, changes to the structure, the wing is completely different, the entire W&B is completely different. It's a heavier, more robust aircraft because it's dealing with more weight and a lot more power. You cannot simply say "I'll take the power output of an LSA Rotax powered T-51, but put it on the airframe with the all of the weight of one designed to fly with literally 4x the power.Now my point here is that this airplane was originally designed for a 100hp 140lb engine. It has had Suzuki and Honda 6 cylinder engines installed in versions of it and now is working toward an appx 400hp 600lb LS3. I think it demonstrates there is a wide spectrum of engine choices which would work in not only the Titan but most any other homebuilt. The key criteria before making any choice is weight. I don't care what any engine is capable of power wise, first it must meet the maximum weight the cg can handle. So, someone who is building a similar airplane but doesn't have the deep pockets of a TFG could build a perfectly acceptable airplane with a stock 430hp LS3 and have 160 hp @ 2400 and 220 hp @ 3200 and use a 2 or 3 blade prop.
Without increasing weight a stock LS3 480 would supply 250hp @ only 3000 rpms. Now any airplane that can carry the weight of an LS3 can have 250hp @ a usable 3000 rpms. Why would most builders need a reduction drive when very very few would ever need more than the LS can supply usuably in direct drive. Expense is cut way down. Weight is lessened. There are fewer parts to fail and less chance of a harmonic issue.
If someones airplane cannot handle the weight then they move to a lighter and less powerful direct drive or a much lighter and smaller engine with a redrive. You have to compare wt/hp available whether its a redrive or not...THEN compare cost and complexity. There is a broad range from 180hp to 300 hp where a direct drive LS engine fits in just fine.........but the popular thing seems to be buying a redrive for all LS conversions rather than just when needed.
So the idea that "very very few would ever need more than the LS can supply usuably in direct drive" is, I dunno, a total fallacy? An aircraft with 4-500 hp might indeed be a hotter ship than one designed for 120-180hp, but taking an aircraft tuned to fly at 4-500hp and giving it only 180 doesn't make it better for people who can't handle a hotrod. It just makes it likely dangerously underpowered for its bulk. If you want an aircraft made for 180hp or less as a 'common man's easy to fly beast' then it needs to stay light so that it still can climb out and perform with that power range. I'd say you want an engine under 300lbs.
Second, I can't follow all your math comparing things here but, your weight delta between an LS3 direct drive with 30lbs of front end bell housing, and an LS3 with a 85lb gearbox is, if I'm interpreting things, 55lbs. For a difference in performance of 250+ hp.
In my world of direct-drive radials (I'm all for the merits of direct drive BTW) if I could gain 10-20 hp with the addition of 10-20lbs I'd do it. Easy choice. If I could add a 80-lb gearbox and make a 9-cylinder radial that puts out 160hp into one that generates 250hp I'd still call that an absolute win. But these radials aren't designed to run any faster than their current RPM range so the gearbox gains me almost nothing. The LS engines are absolutely meant to go to higher RPM and so kneecapping it doesn't serve any good purpose.
The only valid reason to choose an LS engine and not put a gearbox on, is straight up cost. It's dirt cheap dirty power. If you want safety and simplicity, get an O-540. If you want performance, get a gearbox. If you want low cost... I guess an LS maybe? Or find a used O-540 or even an IO-360 or similar for probably about the same price.
Only other case is if you have an actual 1930's era design you're building an experimental replica of, that indeed is designed for flying around on 200hp with a 450lb engine, and you don't want the risks and costs of rebuilding an actual old motor.