I crafted a Unique Variant 4* Ascended Calibur which is now Level 10 and from the looks of it the attack bar has reached its maximum (or virtually so). The idea of upgrading it to Leviathan is appealing, but checking its attack bar it seems almost identical to my 4* Calibur. Is this possible? Does that mean that upgrading it is pointless since the attack power will be the same? Or will I get a UV Leviathan which is even better than both my UV Calibur and a standard Leviathan?
Thanks!
First, there is no way that you got UV "Increased Normal Damage" on a weapon! (same goes with any kind of damage, elemental/shadow/piercing etc)
Unique Variants on weapons can either be Increased Damage against certain type of monsters (ex. Damage Bonus vs Beast) , Increased Attack Speed, or Charge Time Reduction.
So check what UV you got, and closely to your Ascended, and you will see that the bar is not actually increased!
For the second part, about upgrading it to Levi. Of course do that! When you have an item with UV on, and want to upgrade it you have the option to keep the UV on!
So if you got an "Attack Speed Increased Medium" Ascended Calibur you will craft an "Attack Speed Increased Medium" Leviathatn Blade!
You wont lose the UV when you upgrade it, unless you choose to do so.
To clarify further, by default, the game keeps any UV that you have on an item when you upgrade it. When you choose to forsake an existing UV when upgrading, the game rerolls your UVs entirely, both their chance for happening at all, as well as what bonus it might have if it does in fact spawn with a UV. So, if you have a 1 or 2 star item with a UV that you do not care about (Low Increased damage to slimes on a piercing weapon? Blech.) You can scrap it for another chance when upgrading it. However, if you get Very High Charge Time Reduction on a Brandish, you want to keep that UV all the way to 5 star.
See for yourself, the wiki has damage tables for both weapons:
http://wiki.spiralknights.com/Ascended_Calibur#Damage
http://wiki.spiralknights.com/Leviathan_Blade#Damage
Not sure I understand the UV question, but when you upgrade a weapon you have the option of keeping the UV. So if you have an Ascended Calibur with a UV then you can upgrade it to Leviathan and it'll keep exactly the same UV.