For right now, if you have used the Deband and Shifta lv10 discs, it's the best you are going to get.
Shifta and Deband add PWR and DEF based on Base Stats (Natural Stats+Mag Stats+Stat-Up Skills), not based on your Total Stats (Base Stats+Weapon+Units+Abilities).
At level 10, Shifta will add 18.9% PWR, while with Advance Shifta maxed out it adds ~23.63%PWR.
On the Techer Tree, the biggest Shifta-based benefit is Shifta Strike since that does just give a flat 10% Damage Bonus when maxed. As for Deband, Deband Cut and Deband Toughness are what you want to have maxed out since that is a flat 15% Damage reduction and a 25% HP increase, both of which have a great impact on keeping yourself and your team alive.
As for range, that's just the way that Support Techs work in this game. Range Extender adds 2 meters to the range of your uncharged Support Techs and 4 meters when they are charged; for reference Shifta's natural range is ~1.5 meters uncharged and 6 meters charged.
Eventually, they will add higher level discs (up to lv17) that increase Shifta/Deband's base power, and Tech customization which will allow you to have an bigger range on-top of Range Extender (+ 1 or 2 meters depending on the Recipe). Also, once we get to lv85, Techter has a Skill that lets you apply Shifta and Deband to your own party from any distance.